I've created the sketches for a lofted boss that is circular at one end, square at the other end and is supposed to curve along a centerline.  It previews as it should, but says that "The feature could not be created because it would produce self-intersecting geometry" when I click the green check mark.

I made this work once, but it was on the wrong side of the part (mirrored) and I have not been able to make it work again since then.  What you see was built from scratch so I could eliminate interaction with other geometry on the part as a cause.  It will give me an uncurved loft, but balks when I select the centerline.

There does not seem to be much space for a conflict to hide in on this one, so I am puzzled.  Thanks for looking!