[horde] No Thunderbird-Sync since last update

[Michael Polenske]

Good morning,

I use Horde Groupware Webmail Edition 5.1.3 at home. Yesterday I saw  
that a new release of imp was available (6.1.6 instead of 6.1.5). So I  
updated my horde installation via pear-command ("pear upgrade -a -B -c  
horde").

Since then I cannot sync my Thunderbird-Calendar with horde anymore.  
TB is in an endless-loop requesting username/password. On horde-server  
I see this:

--------------------snipp--------------------
2014-02-27T11:33:31+01:00 DEBUG: HORDE Load config file (conf.php;  
app: horde) [pid 7721 on line 402 of "/usr/share/php5/PEAR/Horde.php"]
2014-02-27T11:33:31+01:00 DEBUG: HORDE Horde_Registry: retrieved app  
with cache ID  
horde_registry|app|1383147983|00e8fe001c2c2aad4d44e1f9c0a143f5 [pid  
7721 on line 2027 of "/usr/share/php5/PEAR/Horde/Registry.php"]
2014-02-27T11:33:31+01:00 WARN: HORDE PHP ERROR: Invalid argument  
supplied for foreach() [pid 7721 on line 61 of  
"/usr/share/php5/PEAR/Horde/Core/Factory/Notification.php"]
2014-02-27T11:33:31+01:00 DEBUG: HORDE 1. Horde_Registry::appInit()  
/srv/www/secure/hordeweb5/rpc.php:79
2. Horde_Registry->__construct() /usr/share/php5/PEAR/Horde/Registry.php:279
3. Horde_Core_Factory_Notification->addApplicationHandlers()  
/usr/share/php5/PEAR/Horde/Registry.php:563
4. Horde_ErrorHandler::errorHandler()  
/usr/share/php5/PEAR/Horde/Core/Factory/Notification.php:61
--------------------snipp--------------------

Via Web-Interface and Active-Sync (Apple Device) Calendar-Access is no  
problem.

Any hint/idea what went wrong ?

Cheers - Michael