Hi alteschinken, there are 2 objects at the cutting location, one is a rectangle curve (this one):



And the other is a plane surface:



You'll get different results if you select the either the curve or the surface as the cutting object, because planar curves will make a cut by extruding out a cutting volume like this:



If you select the plane as the cutting object and not the curve it should work as you wanted.

Please let me know if you're still seeing a problem there.

- Michael