News!
I forced disable VSYNC in the driver options. With VSYNC disabled, the donut demo renders glitchless! So the problem seems to be connected to vsync.
Unreal is clean (besides some expected tearing because of disabled vysnc). Only chasm is still unplayable, but it seems a little less distorted than with vsync activated. Looks like some synchronizing signal is not connected somehow, and disabeling vsync is a workaround in some cases. I measured the VSYNC-Pin on the IC, it is clearly connected to its trace and it is not shorted to neighbouring pins. So that's not the issue.