Here is the only backtraces I see (after eliminating some others which 
were all red herings). This backtrace comes at the very beginning of a 
CalDaV session and only once:

```
2019-05-12T00:48:57+02:00 EMERG: HORDE Invalid VObject, line 1 did not 
follow the icalendar/vcard format [pid 17266 on line 143 of 
"/usr/share/php/Sabre/VObject/Reader.php"]
2019-05-12T00:48:57+02:00 DEBUG: HORDE  1. 
Horde_Rpc_Webdav->getResponse() /usr/share/horde/rpc.php:160
  2. Sabre\DAV\Server->exec() /usr/share/php/Horde/Rpc/Webdav.php:66
  3. Sabre\DAV\Server->invokeMethod() /usr/share/php/Sabre/DAV/Server.php:214
  4. call_user_func() /usr/share/php/Sabre/DAV/Server.php:474
  5. Sabre\DAV\Server->httpReport()
  6. Sabre\DAV\Server->broadcastEvent() 
/usr/share/php/Sabre/DAV/Server.php:1066
  7. call_user_func_array() /usr/share/php/Sabre/DAV/Server.php:433
  8. Sabre\CalDAV\Plugin->report()
  9. Sabre\CalDAV\Plugin->calendarQueryReport() 
/usr/share/php/Sabre/CalDAV/Plugin.php:265
10. Sabre\CalDAV\Calendar->calendarQuery() 
/usr/share/php/Sabre/CalDAV/Plugin.php:591
11. Sabre\CalDAV\Backend\AbstractBackend->calendarQuery() 
/usr/share/php/Sabre/CalDAV/Calendar.php:372
12. Sabre\CalDAV\Backend\AbstractBackend->validateFilterForObject() 
/usr/share/php/Sabre/CalDAV/Backend/AbstractBackend.php:119
13. Sabre\VObject\Reader::read() 
/usr/share/php/Sabre/CalDAV/Backend/AbstractBackend.php:147
14. Sabre\VObject\Reader::readLine() 
/usr/share/php/Sabre/VObject/Reader.php:70

2019-05-12T00:48:57+02:00 DEBUG: HORDE [horde] Max memory usage: 
58720256 bytes [pid 18996 on line 613 of 
"/usr/share/php/Horde/Registry.php"]
2019-05-12T00:48:59+02:00 EMERG: HORDE Allowed memory size of 
536870912 bytes exhausted (tried to allocate 467668992 bytes) [pid 
17266 on line 130 of "/usr/share/php/Horde/ErrorHandler.php"]
2019-05-12T00:48:59+02:00 DEBUG: HORDE 1. 
Horde_ErrorHandler::catchFatalError()

2019-05-12T00:48:59+02:00 WARN: HORDE PHP ERROR: Cannot modify header 
information - headers already sent [pid 17266 on line 0 of "Unknown"]
2019-05-12T00:48:59+02:00 DEBUG: HORDE 1. Horde_ErrorHandler::errorHandler()
```

Then I see similar log passages for each event that's being exchanged 
between client and server. Some look like this:

```
2019-05-11T23:41:00+00:00 DEBUG: HORDE Load config file (conf.php; 
app: horde) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE Load config file (nls.php; app: 
horde) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE [horde] Horde_Rpc::__construct 
complete [pid 25876 on line 45 of "/usr/share/php/Horde/Rpc/Webdav.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE Horde_Rpc::authorize() starting 
[pid 25876 on line 137 of "/usr/share/horde/rpc.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE Load config file (prefs.php; 
app: horde) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE Load config file (conf.php; 
app: horde) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:00+00:00 DEBUG: HORDE Load config file (conf.php; 
app: ) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:01+00:00 DEBUG: HORDE [kronolith] Load config file 
(conf.php; app: kronolith) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:41:01+00:00 DEBUG: HORDE [kronolith] Load config file 
(prefs.php; app: kronolith) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 25876 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 25876 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 25876 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 25876 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 25876 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 25876 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 25876 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 25876 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 25876 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Load config file 
(prefs.php; app: horde) [pid 25876 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Fetching stamp WITH token. [pid 25876 on line 252 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Cached.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Synchronized data 
cache for INBOX/Kalender. [pid 25876 on line 63 of 
"/usr/share/php/Horde/Kolab/Storage/Synchronization/OncePerSession.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] INBOX/Kalender has 
30 objects [too many to list]. [pid 25876 on line 114 of 
"/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Fetching stamp WITH token. [pid 25876 on line 252 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Cached.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Incremental folder sync: user: m.gabriel@das-netzwerkteam.de, folder: 
INBOX/Kalender, last_sync: 1557618061, current_sync: 1557618061, 
changes: Array
(
     [add] => Array
         (
             [5319] => 59ad2597-807c-441b-89ef-01bb94fbc969
         )

     [del] => Array
         (
             [5318] => 59ad2597-807c-441b-89ef-01bb94fbc969
         )

)
  [pid 25876 on line 351 of "/usr/share/php/Horde/Kolab/Storage/Data/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Incremental Horde_History update for user: 
m.gabriel@das-netzwerkteam.de, folder: INBOX/Kalender, prefix: 
kronolith:YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzOjg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ: [pid 25876 on line 73 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Cache.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Modifying object in Horde_History: 
kronolith:YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzOjg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ:59ad2597-807c-441b-89ef-01bb94fbc969, uid: 5319, force: 0 [pid 25876 on line 127 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Base.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Object still existing: object: 59ad2597-807c-441b-89ef-01bb94fbc969, 
vanished IMAP uid: 5318. Skipping delete from Horde_History. [pid 
25876 on line 73 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Cache.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] Modified data 
object 59ad2597-807c-441b-89ef-01bb94fbc969 in INBOX/Kalender. [pid 
25876 on line 409 of "/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE [kronolith] INBOX/Kalender has 
30 objects [too many to list]. [pid 25876 on line 114 of 
"/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:41:01+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 25876 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-12T01:41:01+02:00 DEBUG: HORDE Max memory usage: 2097152 bytes 
[pid 25876 on line 613 of "/usr/share/php/Horde/Registry.php"]
```

... and some look like this:

```
2019-05-11T23:40:42+00:00 DEBUG: HORDE Load config file (conf.php; 
app: horde) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE Load config file (nls.php; app: 
horde) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE [horde] Horde_Rpc::__construct 
complete [pid 17266 on line 45 of "/usr/share/php/Horde/Rpc/Webdav.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE Horde_Rpc::authorize() starting 
[pid 17266 on line 137 of "/usr/share/horde/rpc.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE Load config file (prefs.php; 
app: horde) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE Load config file (conf.php; 
app: horde) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE Load config file (conf.php; 
app: ) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE [kronolith] Load config file 
(conf.php; app: kronolith) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-11T23:40:42+00:00 DEBUG: HORDE [kronolith] Load config file 
(prefs.php; app: kronolith) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 17266 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 17266 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 17266 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 17266 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 17266 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 17266 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieving namespaces. [pid 17266 
on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Driver 
"Horde_Kolab_Storage_Driver_Imap": Retrieved namespaces 
[Horde_Kolab_Storage_Folder_Namespace_Imap: "INBOX" (personal, "/"), 
"user" (other, "/"), "" (shared, "/")]. [pid 17266 on line 310 of 
"/usr/share/php/Horde/Kolab/Storage/Driver/Decorator/Base.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] REQUEST OUT IMAP: 0 
ms [getNamespace] [pid 17266 on line 276 of 
"/usr/share/php/Horde/Kolab/Storage/List/Tools.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Load config file 
(prefs.php; app: horde) [pid 17266 on line 109 of 
"/usr/share/php/Horde/Registry/Loadconfig.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Fetching stamp WITH token. [pid 17266 on line 252 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Cached.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] Synchronized data 
cache for INBOX/Kalender. [pid 17266 on line 63 of 
"/usr/share/php/Horde/Kolab/Storage/Synchronization/OncePerSession.php"]
2019-05-12T01:40:42+02:00 DEBUG: HORDE [kronolith] INBOX/Kalender has 
30 objects [too many to list]. [pid 17266 on line 114 of 
"/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Fetching stamp WITH token. [pid 17266 on line 252 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Cached.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Incremental folder sync: user: m.gabriel@das-netzwerkteam.de, folder: 
INBOX/Kalender, last_sync: 1557618042, current_sync: 1557618043, 
changes: Array
(
     [add] => Array
         (
             [5312] => 2B0D064A-CE3C-4125-B8FF-93A0AD004305
         )

     [del] => Array
         (
             [5246] => 2B0D064A-CE3C-4125-B8FF-93A0AD004305
         )

)
  [pid 17266 on line 351 of "/usr/share/php/Horde/Kolab/Storage/Data/Base.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Incremental Horde_History update for user: 
m.gabriel@das-netzwerkteam.de, folder: INBOX/Kalender, prefix: 
kronolith:YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzOjg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ: [pid 17266 on line 73 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Cache.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Modifying object in Horde_History: 
kronolith:YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzOjg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ:2B0D064A-CE3C-4125-B8FF-93A0AD004305, uid: 5312, force: 0 [pid 17266 on line 127 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Base.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] [KOLAB_STORAGE] 
Object still existing: object: 2B0D064A-CE3C-4125-B8FF-93A0AD004305, 
vanished IMAP uid: 5246. Skipping delete from Horde_History. [pid 
17266 on line 73 of 
"/usr/share/php/Horde/Kolab/Storage/Data/Query/History/Cache.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] Modified data 
object 2B0D064A-CE3C-4125-B8FF-93A0AD004305 in INBOX/Kalender. [pid 
17266 on line 409 of "/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE [kronolith] INBOX/Kalender has 
30 objects [too many to list]. [pid 17266 on line 114 of 
"/usr/share/horde/kronolith/lib/Driver/Kolab.php"]
2019-05-12T01:40:43+02:00 DEBUG: HORDE Max memory usage: 2097152 bytes 
[pid 17266 on line 613 of "/usr/share/php/Horde/Registry.php"]
```

Hi,

after I switched to a phone that lacks ActiveSync Support, I tried 
CardDAV/CalDAV against Kolab-Backend, too.

If see the same errors in my logs:

```
2019-05-12T00:35:56+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 3036 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:35:58+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 17266 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:35:58+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 17266 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:35:59+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 17266 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:36:03+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 18996 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:36:03+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 18996 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:36:04+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 18996 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:36:13+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 3043 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:36:13+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 3043 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:36:14+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 3043 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:36:33+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 25712 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:36:33+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 25712 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:36:34+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 25712 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:37:13+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 31135 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:37:13+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 31135 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:37:14+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 31135 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]
2019-05-11T22:38:55+00:00 INFO: HORDE Rewriting userId from 
m.gabriel@das-netzwerkteam.de to m.gabriel [pid 5759 on line 72 of 
"/etc/horde/imp/hooks.php"]
2019-05-11T22:38:55+00:00 NOTICE: HORDE [imp] Login success for 
m.gabriel (Horde user m.gabriel@das-netzwerkteam.de) (37.123.126.38) 
to {imap://mail.das-netzwerkteam.de/} [pid 5759 on line 156 of 
"/usr/share/horde/imp/lib/Auth.php"]
2019-05-12T00:38:56+02:00 ERR: HORDE [kronolith] SQL QUERY FAILED: 
Duplicate entry '59ad2597-807c-441b-89ef-01bb94fbc969' for key 
'index_horde_dav_objects_on_id_internal'
        INSERT INTO horde_dav_objects (id_internal, id_external,
          id_collection) VALUES ('59ad2597-807c-441b-89ef-01bb94fbc969',
          '59ad2597-807c-441b-89ef-01bb94fbc969.ics',
          'YTozOntpOjA7czoyOToibS5nYWJyaWVsQGRhcy1uZXR6d2Vya3RlYW0uZGUiO2k6MTtzO
          jg6IkthbGVuZGVyIjtpOjI7czo1OiJJTkJPWCI7fQ') [pid 5759 on line 318 
of "/usr/share/php/Horde/Db/Adapter/Mysqli.php"]

```

I use Debian buster's set of Horde packages.

Horde 5.2.20
Kronolith 4.2.25-1
Horde_DAV 1.1.4-3

Kolab_Format  2.0.9-3
Kolab_Server 2.0.5-5
Kolab_Session 2.0.3-5
Kolab_Storage 2.2.3-3

I'd really love to see this issue fixed. On the client-side 
counter-part I used SyncEvolution and Thunderbird. With Thunderbird, 
the sync stops on first failure. With SyncEvolution, all events get 
synced, it seems, but I see plenty of http errors on the way and the 
sync never gets marked as successul.

If there is anything I can do to boil this issue down to the 
underlying cause, please let me know.

Is this still happening with recent versions?

Hello,

I've made another Test with Thunderbird - Lightning > Horde.

If i create an Event in Lightning i've got an Error in Lightning:
MODIFICATION_FAILED
Server Replied with 500

Also i've attached the Horde Debug Log-file.
interestingly: After re-sync in Lightning the Event is there and also 
in Kronolith.

Please enable DEBUG logging and include the backtrace.

Hi,
i made some tests with Horde CalDav functionality on Kolab backend.

Turba Contacts works best.
Kronolith gave me an error while create new events.

HORDE: [kronolith] SQL QUERY FAILED: SQLSTATE[23000]: Integrity 
constraint violation: 1048 Column 'id_internal' cannot be null
         INSERT INTO horde_dav_objects (id_internal, id_external,
           id_collection) VALUES (NULL,
           '2074a520-0c2a-4c90-a6c2-6f3b7e3b59fc.ics',
           
'YTozOntpOjA7czoyNDoiZmlybWFAbWVkaWFsLW5ldHdvcmtzLmRlIjtpOjE7czo4OiJLY
           WxlbmRlciI7aToyO3M6NToiSU5CT1giO30') [pid 27184 on line 200 
of "/srv/www/htdocs/horde5/pear/php/Horde/Db/Adapter/Pdo/Base.php"]

This is on:
Horde-5.1.6
Kronolith-4.1.5
Horde_Dav-1.0.4

Please let me now, if you need more information.

Regards

Thomas Nintemann