Hi Armin, there does seem to be some kind of boolean bug going on there.

One thing you can do when you see a boolean not turning out how you would expect is to select all the pieces involved and run the Construct > Curve > Isect command (note under Curve section and not Boolean) which will do an intersection calculation and create the intersection results as curves. If the generated curves of intersection look weird then that can help to show what is going wrong. In this case there seem to be some improper extra intersection curves being generated which are going to mess things up. (Brian - note that yes the object is cut but the result is not entirely correct, the remnants of the cutting pieces are not being left behind as they should be).

If you open your file in MoI v1, you will actually be able to do the boolean properly over there, so you can do that for the moment to finish the part and then paste it back into v2.

I'll investigate what is going on, to see if I can fix it up to work in v2 again.

I have narrowed it down somewhat already, it appears that this pair of surfaces (the inside plane from your cutting piece and the cylinder is from your main body part) are getting an intersection calculated for some reason on the extension of the plane, when it should not give any intersection there:



So that's a starting point for me to investigate.

- Michael