Cutom mesh does not show in PIE and crashes on build

Hi,

Today, I looked a bit into custom meshes and before i scream “There is a bug in the PIE system”, I just want to know if I maybe have missed something.
I create a custom mesh in a blueprint, like so:

It shows up pwerfectly in the editor viewports:

However, In the game, its invisible…
(No, im not looking at the “backside”)

Even a full cube is not showing:

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And when I try to build the lighting, the engine crashes.

Any idea what is going wrong?

Cheers,
Klaus