The message in the error log is:

PHP Deprecated:  preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated in /usr/share/kolab-webadmin/lib/Conf.php on line 179

We get the message when trying to read parameters from kolab.conf which are not set (and probably should have a usable default).

Callstack is get() in line 63 which calls expand() in line 177.