I found a solution for me! 
Obviously I had the wrong OpenXR Version! I have a problem with Win 10 Update KB5005565, it won’t install, but this is a different story.
So thats why OpenXR did not get any update on my system and was on Version 107.2108.5003. I then deinstalled “OpenXR Dev” and “OpenXR”, rebootet and installed it back (OpenXR installed automatically). Then I checked Versions.
Now my system uses Version 107.2109.10010 (my “preview” Version was all the time 108.2109.10009 and still is!).
In MSFS I can now switch again back and forth between 2D and VR!
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