After several clean Windows 8.1 installations, which all crashed some time after login and even installation didn't complete properly, I figured out that my network card is the problem. With WIndows 8 it was working. But with Windows 8.1 I only could boot into safe mode.
My mainboard is an Asus m2n-Sli, which has some nvidia LAN network adapter chip on it.
I could disable it in the bios and voila, Win8.1 is working.
Does anybody know how to fix the problem with this network adapter?
If not, please could someone list this in some hardware incompatibility list, because it's a really nasty issue, which I was only finding with detective work.