Hi Mark, the file you posted only seems to contain one curve in it, did you mean to post a file that had the surface you are having problems cutting with?

But one thing I noticed about the curve you posted is that the ends of it are not in vertical alignment - have you possibly swept that in some way that the upper piece is overlapping back on top of itself making a self intersecting surface? If so then that would be the problem - you'll want to get those ends aligned so that you are not creating a self intersecting result which will not be able to be cut properly.

Also if you are making a circular shape it tends to be better to use Construct > Revolve rather than Sweep around a circle path - Revolve is able to make a 100% precise circular shape, while sweep goes through a fitting process and will be circular within tolerance but will be generally more complex with more points created in the resulting surface.

Here's a screenshot of what looks like a vertical misalignment of the ends (I've added in a vertical line to help show it):



Hope this helps!

- Michael