Hi There

I think also that because the inner surface that you boolean out shares the same surface as the outer you sometimes get a conflict.
If you move the inner profiles .1 mm outside of the outers then you get a clean cut as its not trying to cut a surface from a surface on the ends.
Also you get a better result if the inner and outer curves have a similar number of control points, also Offset can throw up some confused curves with way too many control points , I often rebuild them to simplify it.

You can also sweep the inside curves longer than the outer solid to cut with



cheers
shayne