Hi Frederik, there is a little issue with the rails, they are made up of multiple segments that are just a couple of degrees off from being smooth.

But even without tuning that up I can't seem to repeat the bottom cap failing over here.

Here's what I'm doing, using the v5 beta (https://moi3d.com/beta.htm):

I select the outer curve:



Run Construct > Sweep and pick these as the rails:



That makes this result:



I then repeated that with the inner profile which made a second solid and then boolean differenced them to get the attached model.

Maybe the sharp point in the rails is having some strange effect, I'll post a tuned version of that in a moment.

- Michael