I have a cone made using a Swept Flange feature. I am usually able to cut holes in the part so that they come out with a teardrop shape, like this:

The sketched circle is Extrude-Cut with the Normal cut and Optimize Geometry options ticked.

Now I'm getting the "Operation failed due to geometric condition" error.

If I click the preview button (next to the feature red X) I get the following message:

Any ideas what's wrong? I've done this many times and haven't had an issue until today.