Problem pre-solved:
In short: The problem was the PCIe-Riser, that I used to separate the GPU and its heat from the CPU.Or at least I think that it was the problem, because now it seems to run stably and loads correctly. I just let it run for an hour yesterday evening, but I think it's fine now.
I just didn't come to the solution for so long, because the GPU sent signals just fine when I was in the UEFI or loading some bootable test programs. It seems that the PCIe-Riser just didn't sent any Signals to the GPU, when it loaded Windows... which I don't really get... but ok. As an last option I just plugged the GPU in without the Riser, because it was the last thing, that I didn't check.But like many times Its always the last option, like In my childhood, when I played with my Nintendo DS... the game that I wanted to play was often the last game, that I pulled out of the pocket.
My last question is: How can it be that an PCIe-Riser stops working after 6 hours, without anything changing to it?
Anyway, if it still should be something I'll let you know.