ActiveSync unstable

Hi,

After running ActiveSync with Kolab 3 for several days I experience 
troubles from time to time. My Nokia N9 complains it cannot sync and on 
my Android Tablet a message pops up telling me that the server is not 
available. But the synchronization seems to work and the server is 
available.

I've added

$rcmail_config['activesync_debug'] = true;
$rcmail_config['activesync_log_file'] = 
'/var/log/roundcube/activesync';

to /etc/roundcubemail/main.inc.php but the configured log does not 
appear on the filesystem.

And I see the following error in the apache error log:

[Mon Feb 18 20:47:09 2013] [error] [client X] PHP Warning:  
DOMDocument::createTextNode() expects parameter 1 to be string, array 
given in 
/usr/share/kolab-syncroton/lib/ext/Syncroton/Model/AXMLEntry.php on line 
172

[Mon Feb 18 20:47:09 2013] [error] [client X] PHP Catchable fatal 
error:  Argument 1 passed to DOMNode::appendChild() must be an instance 
of DOMNode, null given, called in 
/usr/share/kolab-syncroton/lib/ext/Syncroton/Model/AXMLEntry.php on line 
84 and defined in 
/usr/share/kolab-syncroton/lib/ext/Syncroton/Model/AXMLEntry.php on line 
172

Sometimes I also see a HTTP return code 500, but I'm not able to see 
the error message. The configured PHP error log 
(/etc/php5/apache2/php.ini) is not showing up, even after restarting 
Apache.

I'm willing to help debug this issue, just tell me what more 
information you need. I'm running Syncroton version 
kolab-syncroton-2.1-rc1 (posted on this ML).

Cheers,
Tobias