Not necessary, this has long been released as a bug fix version.

I think i've got the same Problem, but can't read the commit. Please 
let me know what you have done there.

Tank You

I accidentally a word.

This commit has fixed some things: 
http://horde.myhost.tld/rpc.php/calendars/myuser@myhost.tld now lists 
all my calendars, subscribing to one calendar using Thunderbird (with 
CalDAV) and the following URL: 
http://horde.myhost.tld/rpc.php/calendars/myuser@myhost.tld/calendar:{id of 
the calendar}/

The account URL 
(http://horde.myhost.tld/rpc.php/principals/myuser@myhost.tld/) yields 
an empty directory in my browser and does not work on my Android phone 
(using CalDAV Sync Beta - I can not guarantee that the application 
itself is flawless, but I tested it with DaviCal as CalDAV-Server and 
it worked).

The function Kronolith_Application::davGetCollections() seems to 
return a valid result:
array(2) {
   [0]=>
   array(7) {
     ["id"]=>
     string(32) "calendar:vWinF3HZnIl5NXmL1JvCag1"
     ["uri"]=>
     string(32) "calendar:vWinF3HZnIl5NXmL1JvCag1"
     ["principaluri"]=>
     string(34) "principals/myuser@myhost.tld"
     ["{DAV:}displayname"]=>
     string(4) "Main"
     ["{urn:ietf:params:xml:ns:caldav}calendar-description"]=>
     string(0) ""
     ["{http://apple.com/ns/ical/}calendar-color"]=>
     string(7) "#456eff"
     ["{urn:ietf:params:xml:ns:caldav}supported-calendar-component-set"]=>
     object(Sabre\CalDAV\Property\SupportedCalendarComponentSet)#277 (1) {
       
["components":"Sabre\CalDAV\Property\SupportedCalendarComponentSet":private]=>
       array(1) {
         [0]=>
         string(6) "VEVENT"
       }
     }
   }
   [1]=>
   array(7) {
     ["id"]=>
     string(32) "calendar:i9s9BnlV_pyP-xaCLVOoTw1"
     ["uri"]=>
     string(32) "calendar:i9s9BnlV_pyP-xaCLVOoTw1"
     ["principaluri"]=>
     string(34) "principals/myuser@myhost.tld"
     ["{DAV:}displayname"]=>
     string(8) "BarCraft"
     ["{urn:ietf:params:xml:ns:caldav}calendar-description"]=>
     string(0) ""
     ["{http://apple.com/ns/ical/}calendar-color"]=>
     string(7) "#179000"
     ["{urn:ietf:params:xml:ns:caldav}supported-calendar-component-set"]=>
     object(Sabre\CalDAV\Property\SupportedCalendarComponentSet)#278 (1) {
       
["components":"Sabre\CalDAV\Property\SupportedCalendarComponentSet":private]=>
       array(1) {
         [0]=>
         string(6) "VEVENT"
       }
     }
   }
}

Changes have been made in Git (master):

commit 59aeec910112043594849c85d8bf47ec63b31240
Author: Jan Schneider <jan@horde.org>
Date:   Mon May 27 21:51:35 2013 +0200

     Cast null to array (Bug #12275).

     There is way to determine whether an Horde_Registry_Application 
method exists
     and we don't want to implement these methods as stubs in the base 
class either.

  framework/Dav/lib/Horde/Dav/Calendar/Backend.php |    7 ++-----
  1 files changed, 2 insertions(+), 5 deletions(-)

http://git.horde.org/horde-git/-/commit/59aeec910112043594849c85d8bf47ec63b31240

Not really. This means that the CalDAV/CardDAV server is there (it 
provides the "calendars" and "addressbooks" entries), but it doesn't 
provide the calendars/task lists/address books, even though it uses 
almost the same code like what provides the entries below 
kronolith/nag/turba.
I'm afraid you need to dig inside 
Kronolith_Application::davGetCollections() and see why it doesn't 
return anything, while Kronolith_Api::browse() does.

All the directories Sabre serves (via http://horde.myhost.tld/rpc.php/):

principals -> empty
calendars -> empty
addressbooks -> empty
kronolith
     myuser@myhost.tld
         vWinF3HZnIl5NXmL1JvCag1 (Collection, contains lots of 
text/calendar files)
        vWinF3HZnIl5NXmL1JvCag1.ics
        i9s9BnlV_pyP-xaCLVOoTw1 (Collection, contains lots of text/calendar files)
        i9s9BnlV_pyP-xaCLVOoTw1.ics
turba
     global
         favourites -> empty
     myuser@myhost.tld
         4gRMs8YWEopql_NaGpd2FA6 -> empty
nag
     myuser@myhost.tld
         4XBVL6tfUb7OfI2Riwy0XQ1 -> empty
         4XBVL6tfUb7OfI2Riwy0XQ1.ics

I have two calendars, they seem to show up in kronolith/myuser@myhost.tld.

Hope this helps

And does the calendar show up when browsing to 
http://horde.myhost.tld/rpc.php/calendars/ with the browser manually?

WebDAV/ICS works as expected, Feed yields an empty result. No errors 
in any log file.

Do the other subscription/export links work for that calendar?

Configuration says I have 4.0.2:

Tasks (nag) 4.0.2

Any chance you have an older version of Nag installed?

Sabre responds:

<d:error>
   <s:exception>Sabre\DAV\Exception</s:exception>
   <s:message>Method does not exist.</s:message>
   <s:sabredav-version>1.8.4</s:sabredav-version>
</d:error>

Please try this and report back.

Changes have been made in Git (master):

commit d14869ecd02f8800e63f63a86de6ed27e328ef1b
Author: Jan Schneider <jan@horde.org>
Date:   Mon May 27 20:42:33 2013 +0200

     Throw exception on error (Bug #12275).

  framework/Dav/lib/Horde/Dav/Calendar/Backend.php |    5 ++++-
  1 files changed, 4 insertions(+), 1 deletions(-)

http://git.horde.org/horde-git/-/commit/d14869ecd02f8800e63f63a86de6ed27e328ef1b

CalDAV synchronisation does not work with my clients (Thunderbird, 
CalDAV Sync Beta on Android).
I have installed everything according to instructions and then 
upgraded kronolith to kronolith-beta (using pear, it now is at version 
4.1.0beta2).

The subscription URL for my calendar is (I have redacted my host 
information): 
http://horde.myhost.tld/rpc.php/calendars/myuser@myhost.tld/calendar:vWinF3HZnIl5NXmL1JvCag1/

Opening this URL in my browser prints a Sabre-Exception: "Calendar 
with name 'calendar:vWinF3HZnIl5NXmL1JvCag1' could not be found" and 
the following lines in my syslog:

May 25 15:33:31 x HORDE: [horde] PHP ERROR: array_merge(): Argument #2 
is not an array [pid 20279 on line 92 of 
"/usr/share/php/Horde/Dav/Calendar/Backend.php"]
May 25 15:33:31 x HORDE: [horde] PHP ERROR: Invalid argument supplied 
for foreach() [pid 20279 on line 164 of 
"/usr/share/php/Sabre/CalDAV/UserCalendars.php"]
May 25 15:33:31 x HORDE: [horde] PHP ERROR: array_merge(): Argument #2 
is not an array [pid 20279 on line 92 of 
"/usr/share/php/Horde/Dav/Calendar/Backend.php"]
May 25 15:33:31 x HORDE: [horde] PHP ERROR: Invalid argument supplied 
for foreach() [pid 20279 on line 164 of 
"/usr/share/php/Sabre/CalDAV/UserCalendars.php"]

The account subscription URL 
(http://horde.myhost.tld/rpc.php/principals/myuser@myhost.tld/) 
doesn't work either, altough no errors appear in syslog.

Regards,
Fabian