It seems that this might have already been fixed in GIT. No errors 
have occured in E7 after this change:

Horde/SyncMl/Backend/Horde.php:
110c110
<             $this->_logger->logMessage(
---

Horde SyncML synchronizations fail after the first one in "Connection 
error" message on Nokia E7 phone. This bug may be affecting also Nokia 
E72 (can't get it to work either).

Setup:
Phone is Nokia E7.
Horde is installed to a apache virtual host, using postgresql database.
Relevant version numbers from pear listing:
horde/content                   1.0.2    1.0.2 Tagging application
horde/groupware                 4.0.2    4.0.2 Horde Groupware
horde/horde                     4.0.8    4.0.8 Horde Application Framework
horde/Horde_SyncMl              1.0.4    1.0.4 Horde_SyncMl provides 
an API for processing SyncML requests
horde/Horde_Xml_Wbxml           1.0.1    1.0.1 Horde_Xml_Wbxml 
provides an API for encoding and decoding WBXML documents used in 
SyncML and other wireless applications.
horde/kronolith                 3.0.7    3.0.7 A web based calendar
horde/mnemo                     3.0.2    3.0.2 A web based notes manager
horde/nag                       3.0.3    3.0.3 A web based task list manager
horde/timeobjects               1.0.2    1.0.2 Horde timeobjects application
horde/turba                     3.0.5    3.0.5 A web based address book

How to reproduce:
1. Purge all contacts from horde.
1. Import test.syncml.vcards.txt to horde address book. Use settings 
vcard, replace existing address book, charset iso-8859-1.
2. Clear all syncml sessions data from horde.
3. Purge all contacts from phone.
4. Configure phone for horde SyncML synchronization. Use server 
version 1.1. Include items to synchronize: Contacs, Calendar, Notes.
5. Start normal sync (Synchronize) from phone.
6. Horde sends contacts to phone.
7. Do another normal sync from phone.
8. Synchronization ends in "Connection error".

Attached is horde_syncml_debug.tar.gz containing:
horde_log: Complete Horde INFO log for both syncs.
test.syncml.vcards.txt: Test vcards imported to Horde
sync/: The Horde synchronization debug directory containing datas for 
both first and second sync.

/tmp/sync contents after 1st sync (packetnum.txt contains number 20):
-rw------- 1 www-data www-data    471 2011-08-17 02:08 client_10.wbxml
-rw------- 1 www-data www-data   4234 2011-08-17 02:08 client_11.wbxml
-rw------- 1 www-data www-data  19639 2011-08-17 02:09 client_12.wbxml
-rw------- 1 www-data www-data  19595 2011-08-17 02:09 client_13.wbxml
-rw------- 1 www-data www-data   4980 2011-08-17 02:09 client_14.wbxml
-rw------- 1 www-data www-data 152509 2011-08-17 02:09 data.txt
-rw------- 1 www-data www-data  27688 2011-08-17 02:08 devinf.txt
-rw------- 1 www-data www-data 107148 2011-08-17 02:09 log.txt
-rw------- 1 www-data www-data      2 2011-08-17 02:09 packetnum.txt
-rw------- 1 www-data www-data    997 2011-08-17 02:08 server_10.wbxml
-rw------- 1 www-data www-data  39862 2011-08-17 02:08 server_11.wbxml
-rw------- 1 www-data www-data  39665 2011-08-17 02:09 server_12.wbxml
-rw------- 1 www-data www-data   9918 2011-08-17 02:09 server_13.wbxml
-rw------- 1 www-data www-data    371 2011-08-17 02:09 server_14.wbxml

/tmp/sync contents after 2nd sync (packetnum.txt contains number 21):
-rw------- 1 www-data www-data    471 2011-08-17 02:08 client_10.wbxml
-rw------- 1 www-data www-data   4234 2011-08-17 02:08 client_11.wbxml
-rw------- 1 www-data www-data  19639 2011-08-17 02:09 client_12.wbxml
-rw------- 1 www-data www-data  19595 2011-08-17 02:09 client_13.wbxml
-rw------- 1 www-data www-data   4980 2011-08-17 02:09 client_14.wbxml
-rw------- 1 www-data www-data 152509 2011-08-17 02:09 data.txt
-rw------- 1 www-data www-data  27688 2011-08-17 02:08 devinf.txt
-rw------- 1 www-data www-data 107148 2011-08-17 02:09 log.txt
-rw------- 1 www-data www-data      2 2011-08-17 02:09 packetnum.txt
-rw------- 1 www-data www-data    997 2011-08-17 02:08 server_10.wbxml
-rw------- 1 www-data www-data  39862 2011-08-17 02:08 server_11.wbxml
-rw------- 1 www-data www-data  39665 2011-08-17 02:09 server_12.wbxml
-rw------- 1 www-data www-data   9918 2011-08-17 02:09 server_13.wbxml
-rw------- 1 www-data www-data    371 2011-08-17 02:09 server_14.wbxml