[horde] logout issues

[Hans Kind]

Horde: 3.1.5
Imp: H3 (4.1.5)
PHP Version: 5.2.5

We see a strange behavior on 1 of our servers with Horde installed.

Login is no problem, but after 2-3 minutes, user is "redirected" back 
to the login screen, or is presented with a blank screen.

This started 2 day's ago, before it was working ok, and there have 
been no updates installed just before this change in behavior.

Any suggestion on what might be going on!

rgds

Hans Kind