Hi Jurgen, I'm not 100% certain what's going on there, but it may have something to do with these areas of your model where you've got opposite sides of the model coming down to meet each other flatly:



Having that area collaping down exactly to the same tangent there may not be a good thing, because it means there is a small area where the upper and lower surfaces are overlapping each other, and self-intersections between different surfaces in a solid is not a good thing to have.

You might have some better success if the curvy piece ended with a little bit of an angled tangent leaving some small volume area in those tips rather than having the curvy piece come down to exactly the same tangent direction as the straight piece making for a totally flattened area there.

- Michael