Hi Sven,

Perhaps I'm wrong, but, when I open your file, the cylinder is a solid but seems having missing part or invisible part between its cap and the rest of cylinder.

Normal ?

nothing hidden but an hole.

And this "hole" is a surface too. Strange surface because it has 2 edges and those edges are 1 curve and 1 closed curve.

Having this type of surface is potentialy a source of strange thing.