okay, after days of frustration I figured it out. I started to inspect every warning in the Output Log and tried to get rid of it. One warning was: “LogD3D12RHI: Warning: Failed to query NVIDIA driver version”.
I then searched in source code to find the code and reproduced the call to NvAPI_SYS_GetDriverAndBranchVersion(&DriverVersion, BranchString); and found that the return value was NVAPI_NO_IMPLEMENTATION which sounded strange. I googled a bit and found something connected to RenderDoc (totally different problem) and I remembered that I had installed RenderDoc-Plugin a few days ago to investigate some rendering stuff. Uninstalling RenderDoc solved it for me, yay.