Wierd Material Behaviour

Dear Unity-Community,

I'm working on a project and there occured a strange behavior on one of my model materials.
All marked materials on these pictures are the same material. Sun comes from behind wall 3.
As you can see, some materials are black, like they should be, others are greyish. It doesn't matter, if they are in light or not, see Object 4 or Wall 3/1. I made all models with Blender Cycles, unwraped them there, gave them Nodematerials. Imported them as obj, gave them the shown material.

I really don't understand, what is happening here, please help me :/