I am trying to add an outline around this geometry, that is overlapping a plane. This is not the result I was hoping for. I thought the checkbox "Material Boundary" would solve my problem but it doesn´t really seem to do anything. Please help D: (using the newest version of blender)
This is not an error. Unfortunately, freestyle cannot detect face intersections, even if they have different materials. You can use inset face or a loop cut and slide to try and put an edge there for freestyle to use, but unless there is an edge there or an outline, there will be no freestyle.