Hi Gunter, so the problem there is that a line that is angled diagonally in 3D space does not have a well defined single extrusion direction.

The boolean will want to extrude the line into a plane and then use the plane to cut the object, but it needs to know what direction it should extrude it. When your line is on one of the world axis planes it will use that axis direction, that's why those other lines worked.

For your situation with a line at an angle not in a world plane like that, you will need to extrude the line into a plane yourself and then use the plane as the cutting object in the boolean. There is a "Set dir" option in the Extrude command that can be used for extruding in a specific direction.

Another way that might work more easily for making an angled cutting plane is drawing it using the Draw solid > Plane > 3 pts command.

- Michael