Re: Double log-in: Possible security threat!
Okay, I could resolve this. I dug deep into the Forum and found an entry "mobile theme broke my Wiki", and that may be the clue. I set the theme to something entirely different (both default and user theme), and voilá, login works again. Then I separated old and new Wiki by renaming PHPSESSID to PHPSESSID-mywikinname and now they are really really separated and both are functional.
Now my only issue is that I have to work with a priviledged user because my admin is powerless, but that's another Forum entry of mine.