apache frontend not working with ispcp

[fpwgmbh]

I have a working ispcp 1.07 installation on Ubuntu 10.04.1 LTS.
Additionally I have a second server with Ubuntu/Apache installed as a frontend for the ispcp server. A simple rewrite rule should redirect incoming traffic from the frontend to the ispcp server. The rewrite is working too, but:

When I click on http://frontend-server/ispcp/admin/settings.php within the menu the logfile shows it first calls http://ispcp-server/ispcp1/admin/settings.php correctly (ispcp1 is an alias) but then it calls admin/index.php too.
The strange thing about is that when entering the same url (http://frontend-server/ispcp/admin/settings.php) into the browser it does work.

My idea is that has something to do with session/login checking. Has anybody some experience/ideas about using a frontend with ispcp?
(I have done the same with an vhcs installation without a problem.)