So I finally managed to solve the Problem. Also the problem related to Unreal 4 Game on Steam doesn't work with Win11 - #8 by ProbyTheProbe
The problem was the initialization of widget blueprints in c++ which I did a couple of years ago. SInce I did this at 2 different places it was really hard to find the problem. Especially so since everything worked fine besided the packaged exe.
Also doing this with blueprint actors doesn’t seem to be a problem.
Long story short I changed the way i initialize the widgets and now everything is working well.
This was btw the way that caused the problems.
Cheers,
Proby