Task List / Calendar errors

Hi,

I just discovered that the Task List and Calendar in Roundcube is not 
able to save to the backend. Looking at the error log, I see the 
following errors:

[04-Feb-2013 15:35:18,000000 +0000]: PHP Error: Failed to load plugin 
file /usr/share/roundcubemail/plugins/kolab_core/kolab_core.php in 
/usr/share/roundcubemail/program/include/rcube_plugin_api.php on line 
215 (POST /roundcubemail//?_task=tasks&_action=task?_task=&_action=)
[04-Feb-2013 15:35:18,000000 +0000]: PHP Error: Failed to load plugin 
file /usr/share/roundcubemail/plugins/listcommands/listcommands.php in 
/usr/share/roundcubemail/program/include/rcube_plugin_api.php on line 
215 (POST /roundcubemail//?_task=tasks&_action=task?_task=&_action=)
[04-Feb-2013 15:35:18,000000 +0000]: PHP Error: Failed to load plugin 
file 
/usr/share/roundcubemail/plugins/threading_as_default/threading_as_default.php 
in /usr/share/roundcubemail/program/include/rcube_plugin_api.php on 
line 215 (POST 
/roundcubemail//?_task=tasks&_action=task?_task=&_action=)
[04-Feb-2013 15:35:18,000000 +0000]: PHP Error: Failed to load plugin 
file /usr/share/roundcubemail/plugins/contextmenu/contextmenu.php in 
/usr/share/roundcubemail/program/include/rcube_plugin_api.php on line 
215 (POST /roundcubemail//?_task=tasks&_action=task?_task=&_action=)
[04-Feb-2013 15:35:18,000000 +0000]: PHP Error: Error saving task 
object to Kolab server in 
/usr/share/roundcubemail/plugins/tasklist/drivers/kolab/tasklist_kolab_driver.php 
on line 719 (POST 
/roundcubemail//?_task=tasks&_action=task?_task=&_action=)

Any idea what's going wrong? I'm running Kolab 3.0 on Debian Wheezy.

Cheers,
Tobias