Hey ScicoPax,
After some investigation and pulling from various resources, I was able to determine what I believe is the source of your problem. The face orientation for half of your mesh is flipped while the other half is correct. In order to visualize what I mean have a look at the image below.
Faces Orientation
I also imported your mesh into the engine to see if there was a problem occurring there as well. I was able to get some clean lighting on your model by changing some of the ‘Build Settings’.
Build Settings
These are the settings I discovered work well for you model. If you are still having trouble or have additional questions let us know.
Cheers,