Shadows behaving strange (Blender FBX import)

The problem is hard to describe, therefore the screenshot:
On the right: Meshes casting incorrect shadows
On the left: The mesh casting an incorrect shadows upon itself
Some more details about what I tried:
-Lightmap looks like this (As far as I can tell they look alright). For that I made a 2nd UV-channel for my mesh in Blender.

  • Generating a Lightmap leads to worse results
  • Playing around with the Lightmap resolution doesn´t help
  • Pre Building everything is fine
    I have the same problem with other meshes, but for this one it is the most extreme. I have no idea what to do.
    I read this: , but i did not find a clue there.
    Please help me!

Your lightmap is split up too much, you can get better results by doing it by hand so that you can minimize the number of seams. Also, if the mesh is large–with a large surface area, you may need to split up the mesh to get enough detail in the lighting.

Ok, after trying around an entire day in the same level, I now decided to switch levels and everything seems to work fine. It would be still great to know what went wrong there, but at least the mesh seems to be quite ok. Unfortunately the entire level seems to be totally broken :D.
31 Lights cannot be built (although UE doesn´t tell me it has a problem). I guess I try again. Thanks for the help!