Thank you for your answer, the problem is that I have created a room using planes, so the walls, floor and ceiling are one sided. I tried adding a collision hull made out of planes, but they were (for some reason) transformed in the editor too Boxes. Which makes me believe that the editor doesn’t support planes as collision Hulls.
So the real question is: Is it possible to make a collision hull out of planes in your 3D Application? Does Unreal know how to handle a plane as a Collision, or does it have to be an enclosed mesh?