Hi Lars - I see I didn't notice the other break line in your screenshot there.

But it looks like the problem still might be stacked up / duplicated surfaces.

If you use Edit/Separate on that object, and then drag some of the pieces away you'll see that there are actually 4 surfaces in there, not just 2.


Then additionally, after you shell there is a kind of internal wall created in this area:



Use Edit/Separate to break the shell into individual surfaces, then hide the exterior ones and you can see the unwanted interior wall:




And there are actually stacked up pieces on that internal wall there too. I'm not sure yet but this may indicate some kind of loop in the trim curves in that seam area, that may be another problem. I'll take a bit more of a look. Because even deleting those internal wall pieces does not allow the exterior pieces to be joined into a fully closed solid.

- Michael