Thanks a million for your answer. I will try that now. Today, strange enough, when I try to add a comment, the error message is different from yesterday:
An appropriate representation of the requested resource /~id90522/kattenforum/index.php could not be found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
So I don't know what the f*ck is happening here, but I will first try out your solution.