Jette, I'm glad you found a solution.
I had first thought of "Expand stroke" but on the screenshot you can see that it has been applied.
Then I thought it was an error caused by smart nodes but it worked when I tried it out.
Another track was one of the shapes with an open curve (I remember seeing on the official forum a case in which AD had generated by itself a kind of zig-zag visible only with a very very high zoom on a small portion of the curve) but even in this case the operation worked well when I tried to reproduce it.