Hi Gunter,

The problem is caused by the ovals having an edge that is very close to being coincident with the surface of the ring:



I moved one of the ovals that wasn't working away from the ring slightly:



and boolean diff worked fine then.

I don't get the deformed oval like you show. For whatever reason, that one actually worked ok for me. But, it's generally a bad idea to have surfaces or edges just grazing one another for booleans. Just like a surface has edges, some curves have points where they are joined and those points can also cause problems if they are very close to lining up with an edge or a point on the other object in the boolean.

- Peer