So, I found the problem!
@nick
the booting voltage for the vtt is too low! I think, that's the reason, why my mem's won't be dedicated.
I can boot now with vtt1 1.076, vtt2 default, vcssa 0.958.
In windows, it's no problem to lower the voltages! Booting with 2666 needs 1.15 vtt1 and 0.98 vcssa, but in windows, I'm able to lower the settings to 1.08 and 0.96.

Please have that in mind while constructing the next BIOS