This one's really troubling me! I have a group of curves, with which I'm trying to do a solid cutout. If I leave them on the XY plane, it works fine. However, as soon as I rotate them (either as a group or separately), I get the "curves not planar" message and can't complete the action. I've tried ungrouping, regrouping, realigning - I can't get it to work.
I'm about to try rotating the solid that needs the cutout instead, but it would be nice to get to the bottom of this if you have any ideas.
I'm attaching the curves - you'll find that it works as it is, but if you rotate it, it won't.