That's still very little to go on, but I'll take a stab at it.
If "AddPreview.php" is the processing script, then shouldn't the form action be directed to it, i.e "<form action="wiener-wohnungen.net/AddPreview.php...", rather than to "modules.php"? Since you're getting logged out as soon as you submit, this seems like the case. The script "modules.php" maybe requires login before access to it. Try writing a test script as the processor, and do nothing in it except echo the "get" and "post" variables. If that works then chances are it's not a permissions problem (which it shouldn't be anyway since the scripts are all in your home directory).
That's the best I can do with such skimpy information. 🙂