After 2 days now I'm up and running again, but on V5.1 without HF1.
I made many different efforts with HF1, but nothing helped. I made tests for my 2080TI and my CPU, but they showed that my hardware is ok.
At the end I had a Prepar3D.cfg that crashed vanilla 5.1HF1 immediately after starting up into the default scenario (F-35-A at KRND).
I uninstalled everything and installed 5.1 (that is version 5.1.8.26742). That worked. For a test I installed the HF1 client, just the client, and immediately had the DXGI hung error again. Going back to the 5.1 client, everything worked again.
For me this is the only possibility now: stay at 5.1 without HF1.
Karl
p.s.: also ORBX Buildings HD are working again -