2.1 to 2.2-rc-1 upgrade C): horde login impossible

[Johannes Graumann]

When trying to log in at https://morannon.homelinux.org/horde with
credentials perfectly fine from Kontact (aside from the previously reported
freebusy issue), I get the error:
> MessageLogin failed because your username or password was entered 
> incorrectly. 

/kolab/var/apache/horde/horde.log reads parallely:
> Feb 08 22:36:21 HORDE [error] [horde] FAILED LOGIN for <myuser>
> [192.168.0.2] to Horde [pid 19925 on line 146
> of "/kolab/var/kolab/www/horde/login.php"]  

Any hints on what's wrong?

Thanks, Joh