Plesk 12.x / Linux :: Horde Authentication Error - Uncaught Exception

I'm currently experiencing a lot of problems, using Horde. Users are not able to log-in due to this error:

Code:

2014-10-09T10:00:54+02:00 EMERG: HORDE Uncaught exception 'Horde_Exception_PushApp' with message 'User xxx@yyy.tld is not authorized for Horde.' in /usr/share/psa-pear/pear/php/Horde/Registry.php:1555
Stack trace:
#0 /usr/share/psa-pear/pear/php/Horde/Registry.php(1198): Horde_Registry->pushApp('horde', Array)

[Code] .....

I've already tried re-installing Horde, but without success.

There's an article in your KB, but `/var/lib/php/session` does not exist and `/var/lib/php5/sessions` has the required perms.

I've also tried to set session.save_path to `/var/lib/php5/sessions` in `/etc/psa-webmail/horde/horde/php.ini` and after a restart, sessions are written into that directory. But the problem still exists.

Code:
_b|i:1412842564;_r|i:1412864164;horde|a:25:{s:11:"auth/authId";s:13:" xxx@yyy.tld";s:12:"auth/browser";s:110:" Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.101 Safari/537.36";s:16:"auth/credentials";s:4:" imp";s:15:"auth/remoteAddr";s:15:" my.ip.addr.ess";s:11:"auth/userId";s:13:"

[Code] .....