Hello Niq, Hello List, I have been able to solve this issue. Well, I should say, I have found a workaround.
I suspected mod_auth_basic to be doing something wrong, so I had a close look at the sourcecode. It is only run in the check_user_id phase. I tested whether the problem still exists if my module is hooked in at the beginning of the check_user_id phase and returns with status DONE, thus preventing mod_auth_basic to be run at all. The problem is immediately gone, so I'm sure this module is the cause. But I haven't found out what exactly goes wrong. Doesn't matter. This way it works. Thank youfor your time. Greetings -- Christoph Gröver