Hi Tom, it looks like the sweep result has unjoined ("naked") edges inside it.

I believe this is caused by some segments being just barely not tangent to each other, like just a little more than 1 degree off from being tangent.

This makes the sweeper try to do mitered corners which involves extending surfaces and intersecting them. Intersection calculations can tend to be difficult on things that cross each other at a very shallow angle like that because they kind of have something more like a "zone of intersection" rather than a really well defined intersection.

The cure will probably be to tune up control points to make everything that should be tangent to be directly in line with each other.

- Michael