I see. I tried it again, raised those Mesh cards for both the H-profile and Würfel 5 (Würfel = german word for “cube” ^.^ i just made them from a standard cube and didn´t rename them) to 14, like in your screenshot, and now they work here too 
Are there any drawbacks or things to know and consider, if we raise those mesh cards beyond their default value?
And would it be possible, to raise them directly during import into UE5? Because if we know, that the mesh is already complex enough to require more, then an option in the import window might save some extra steps later, especially with mass imports.
Did another test mesh, a more complex one and to my surprise, this one only showed 2 pink surfaces for its complexity. Both were in that front cutout, the botton and top surface. After raising mesh cards to 16, it worked fine too.
The hole in the top center between those two towers worked fine from the beginning.
I guess, we definiately need a good guide for the relations between mesh cards and mesh complexity 
Guess it´s time to dust off some old greeble packs and see, how well they work ^.^