Hi FDP - the bug seems to be something that's happened some time before this step, there is a messed up spot on the base object in at least one spot here:



If you zoom in to that area, it looks like a hole is there, if you then use Edit > Separate on that object and isolate just that one surface and turn on it's surface control points it looks like this:




So whatever generated that surface has done something wrong and has the trimming boundary all the way off the actual surface. Some previous bug kept the wrong part of the surface or something like that.

So at least that particular surface will need to be deleted and reconstructed, maybe others, I'll take a look.

The actual bug is most likely in some previous step that built that piece though.

- Michael