After numerous restarts I can say that I am sure I have found the issue:
Experimental / Multi Window: As I have 3 screens, I have activated the feature. In combination with DX12, this does not work yet.
After going back by deleting the additional windows, changing to “Window” instead of full screen, and dragging the single screen over 3 screens (as before the last months without problem), DX12 and FSR also works on my setup (tested for a while in a flight over NYC).