I had the exact same problem. I took many many hours searching and trying many different things. There is one simple fix for it. Your skybox is set up incorrectly. Your skybox has to be 6 separate pieces, one on each side of the map. And the edges of each skybox side have to be perfectly sealed. So what I did to avoid confusion, was I copied the skybox from the default map by making it a prefab. Add the prefab to your map and stamp it. Then move the pieces to fit your map and seal them, and you should be good. Hope this helps!