Hi Franz, is it possible that one of your objects is not a fully closed solid? When you select them, what does the object type in the upper right area of the window read (in the panel that shows the properties of the current selection) - does it say "Joined srf" or "surface" or "solid".

The result that you show in your screenshot could be the kind of thing that you get when you try to boolean with a not fully closed object, if the intersection does not form a closed loop instead of cutting a spot out it can basically leave an imprint of an inifinitely thin slit instead.

It's hard to know for sure though just by looking at the screenshot - often times if you have a question about a particular situation it can really help if you also post the 3DM model file with actual geometry in it along with your question as well, then it's possible for others to take a look at your objects, zoom in and examine things from different angles, etc...

- Michael