-- Edit Solution : Set unity to rotate around "pivot" instead of "center".
I created an prisma in blender and exported it as .fbx into unity. In the scene Im rotating it via code and I noticed that its rotating around the wrong pivot.
The problem is that I set in blender all objects to the same origin and that should work without problems. But its not working and Im going crazy....
Why I think its the pivot - By using the rotating tool in unity I can see it with my eyes and when rotating the position of the prisma changes.
To rotate my object the right way I used - https://answers.unity.com/questions/319802/rotation-when-importing-from-blender.html
Below you can see both views (looking from top on the prisma), first from Blender, where the origin is in the center of the object and in the second image you see it in unity but the pivot is wrong/changed.