Hi Christian,
It looks like your UV map for the wall is stretched. The default material in UE4 is squares, as seen on the right wall, but on the left wall the squares are elongated into rectangles, which is probably caused by the UVs being stretched. When you apply your wall material, is the issue that the material looks stretched?
The best solution would probably be to edit your UV map in Blender. Here’s some documentation on editing UVs (UV editing) but there are loads of other tutorials as well.
If you need more help, feel free ask, I’m just trying to keep this somewhat concise. If you still have trouble it would also help us if you post a screenshot of your UV map and the wall with the material applied.
Cheers and good luck!
-Noah