Problem with a .blend file and Unity 5 baked GI

Hello there, this problem is driving me crazy. It's almost 2 days that I'm trying to figure this out but I just failed... I have done a 3D model in Blender and I've exported it in Unity. Everything works fine but this:

As you can see, in Blender the floor of this room is just one face, but in Unity it seems that there is a "cut" on it. Very strange. I controlled my model several time, founding nothing. Then I figured out that disabling the Baked GI in the Lighting window will solve this problem, but doing that i loose a lot of atmosphere and realism that I'm looking for my project.

I'm really confused now. I just can't understand what is wrong with my model and the Baked GI! I also tried changing the position of the directional light that is used for the Baked GI, but the cut is still there, just a little bit less stressed. I hope that some of you guys may help me! Cheers! :)