I think, I´ve just found the solution (for me).
Few days ago I changed some things in the Mixed Reality Windows settings, where you can adjust the resolution and performance…
I changed all options where it was possible to “let windows decide”. I now changed it back to “High” and “best visual performance” and now I can switch between VR and Montior again as often as I want.
Hope this helps…
Edit: Hmm na… Had this again now… weird.