[horde] after Update ActiveSync error Did not initialize the PARTIAL sync.

[Steffen]

I have upgrade Horde from

Horde_ActiveSync          2.16.5  stable
webmail                   5.1.4   stable

to

webmail                      5.2.5   stable
Horde_ActiveSync             2.26.0  stable

The configuration says that all databases are updated.

One (the only one that connected after the upgrade) Windows Phone produces 
the log below once a minute for nearly 6 houres now.

Shall I remove the device on the /horde/admin/activesync.php page or 
would this make the problem worse?

syslog:
Feb 27 15:22:57 mailsrv1 HORDE: Returning HTTP 500 while handling Sync command. [pid 8033 on line 158 of "/var/www/horde/rpc.php"]
Feb 27 15:22:57 mailsrv1 HORDE: Error in communicating with ActiveSync server: Did not initialize the PARTIAL sync. [pid 8033 on line 162 of "/usr/share/php/Horde/Rpc/ActiveSync.php"]
Feb 27 15:22:57 mailsrv1 HORDE: #0 /usr/share/php/Horde/ActiveSync/Request/Sync.php(242): Horde_ActiveSync_Collections->getMissingCollectionsFromCache()#012#1 /usr/share/php/Horde/ActiveSync/Request/Base.php(253): Horde_ActiveSync_Request_Sync->_handle()#012#2 /usr/share/php/Horde/ActiveSync.php(882): Horde_ActiveSync_Request_Base->handle()#012#3 /usr/share/php/Horde/Rpc/ActiveSync.php(143): Horde_ActiveSync->handleRequest('Sync', '301087ecd667e65...')#012#4 /var/www/horde/rpc.php(158): Horde_Rpc_ActiveSync->getResponse(NULL)#012#5 {main} [pid 8033 on line 162 of "/usr/share/php/Horde/Rpc/ActiveSync.php"]
Feb 27 15:22:57 mailsrv1 HORDE: Buffer contents:  [pid 8033 on line 162 of "/usr/share/php/Horde/Rpc/ActiveSync.php"]

ActiveSync log:
2015-02-27T15:36:53+01:00 INFO: [10145] Maximum memory usage for ActiveSync request: 16018360 bytes.
2015-02-27T15:36:53+01:00 INFO: [10145] Horde_Core_ActiveSync_Driver::authenticate() attempt for THEUSER
2015-02-27T15:36:53+01:00 INFO: [10145] SYNC request received for user THEUSER
2015-02-27T15:36:53+01:00 INFO: [10145] Device entry exists for THEDEVICEID, updating userAgent and version.
2015-02-27T15:36:53+01:00 INFO: [10145] Request being handled for device: THEDEVICEID, Supporting protocol version: 14.1, Using Horde_ActiveSync v2.26.0
2015-02-27T15:36:53+01:00 INFO: [10145] GET VARIABLES: Array
(
     [ProtVer] => 14.1
     [Cmd] => Sync
     [Locale] => 1031
     [DeviceId] => THEDEVICEID
     [PolicyKey] => 0
     [DeviceType] => WindowsPhone
)

2015-02-27T15:36:53+01:00 INFO: [10145] Handling SYNC command.
2015-02-27T15:36:53+01:00 INFO: [10145] Checking policykey for device: THEDEVICEID user: THEUSER
2015-02-27T15:36:53+01:00 INFO: [10145] Policykey: 0 verified.
2015-02-27T15:36:53+01:00 DEBUG: [10145] I  <Synchronize>
2015-02-27T15:36:53+01:00 DEBUG: [10145] I   <HeartbeatInterval>
2015-02-27T15:36:53+01:00 DEBUG: [10145] I     1500
2015-02-27T15:36:53+01:00 DEBUG: [10145] I   </HeartbeatInterval>
2015-02-27T15:36:53+01:00 DEBUG: [10145] I   <Partial />
2015-02-27T15:36:53+01:00 DEBUG: [10145] I  </Synchronize>
2015-02-27T15:36:53+01:00 INFO: [10145] Executing a PARTIAL SYNC.
2015-02-27T15:36:53+01:00 ERR: No collections in collection handler, loading full collection set from cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded @Contacts@ from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded INBOX from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded @Calendar@ from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded @Tasks@ from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded Sent from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded MAIL1 from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded MAIL2 from the cache.
2015-02-27T15:36:53+01:00 INFO: [10145] Loaded MAIL3 from the cache.
2015-02-27T15:36:53+01:00 ERR: Did not initialize the PARTIAL sync.

-- 
Steffen