Hey Tobias!
Your first .x file didn't open for me.
I took a quick look at the .obj. Some of your vertices where inverted. I fixed the problem and attached a fixed .x file. This
should (didnt test) import flawlessly into FPSC. However, I can't predict what will manifest when you import it in one of these autodesk monstrosities.
You probably hit a hotkey to get these inverted planks, the exporting issues however where likely not your problem. I've seen far worse happen with Mayas and Maxs exporters for less common formats. ...but my personal dislike towards autodesk is probably none of your interest.
The mesh still had some oddly displaying smoothing groups and some vertices that imply that you wanted to add a door or window (?) but didn't go through with it. Its also partially unwrapped (?) in an odd way.
Semi-Fixed .x for FPSC and .obj for anything else attached. Good luck with your further modeling!
-Wolf