Hi Alexander, I was trying to untrim and retrim your model to get rid of the fillet, but I also ran into another problem area with it as well.

This surface here folds back over top of itself right near its ends:



If I separate that surface from the others and then turn on its control points, look at this area here which seems to be bunched up:



If I zoom in on that area, it appears to be in a kind of hook formation:




This kind of surface that folds back over top of itself is particularly bad for shelling or offsetting calculations. The surface normal that the offset tries to track along slams back and forth to opposite sides in a small area there.

It looks like this surface was created as an extrusion, but that the original curve being extruded has that kind of folded over hook at the ends.

Do you happen to have the original curve that was used to create that particular surface?

You want to be careful to make sure the original curve structure that you start with are nice and clean and don't have bad formations like this. If your original curves have bad structures in them, then those will tend to get transferred to surfaces created from those curves as well.

- Michael