Yes, it should show error page when you enter wrong username/password - that is ok.
when I tried register, I got "Das Feld mit dem Bestätigungs-Code muss ausgefüllt werden." - that is NOT ok, because I'm sure entered correct captcha code..
So far I can only speculate that sessions is not correctly setup on the server. It could be wrong path or not writable in session.save_path (see phpinfo)