Hi folks,
I think I got it:
I have created a new project, added a hollow block, placed camera and cube inside and added physics -> works fine.
Since I prefer an open environment, I created a sky cube and removed the walls and roof of the hollow block -> does not work anymore.
Added the roof again -> worked fine.
Looks like the physic objects have to be inside a hollow block.
Is this a bug or a feature?

Always my best,
John