so, going aroun lightdm was not it (the method for that you digged out in the other thread was spot-on btw).
I could not log in to anybody else then root in the tty session neither. it is getting more mysteroius by the minute somehow.
Luckily I am not for my live depending on this machine right now (besides sucking big time).
btw: thx a mil for all the support till here @Mirdarthos & anybody else