[gollem] authorization problem with gollem on horde 5.1.6

[adje at bezoekerscentrumsonsbeek.nl]

  I am installing horde webmail on ubuntu server, using
postfix/dovecot/mysql. I am by no means an expert but so far I have most of
it up and running.
The only application that is giving me headaches is gollem. As soon as I
click on "others > File Manager" I get thrown back at the login screen.
Also the language setting of the login screen is reset to default (English)
at that point.

Logfile:
/2014-04-21T18:44:26+00:00 NOTICE: HORDE [horde] Login success for
adje at bezoekerscentrumsonsbeek.nl [192.168.4.101] to horde. [pid 3302 on
line 150 of "/var/www/horde/login.php"]
2014-04-21T18:44:26+00:00 NOTICE: HORDE [imp] Login success for
adje at bezoekerscentrumsonsbeek.nl (Horde user
adje at bezoekerscentrumsonsbeek.nl) [192.168.4.101] to {localhost:143 [imap]}
[pid 3302 on line 157 of "/var/www/horde/imp/lib/Auth.php"]
2014-04-21T18:46:35+00:00 NOTICE: HORDE User is not authorized for gollem
[pid 3354 on line 306 of "/usr/share/php/Horde/Registry.php"]/

http://myserver/horde/test.php shows no issues for horde or gollem. Looking
at the configuration screen in Horde everything is marked as "Application
is ready" and "SQL DB schema is ready"

I am using Ubuntu 12.04 lts (all up to date), Horde 5.1.6, Gollem 3.0.2

I am a bit lost here. Could somebody point me out in the right direction?

Thanks in advance!
Martin