After the catapult strikes the windmill, it is destroyed and four resources drop: mercury, ore, wood, and gold. Three black squares also appear. After saving the game and then loading it, the entire map is black. The problem is with the resource models. If you delete the resource models, everything works fine. It seems to me that if other resources with modified models dropped from the windmill, it would be the same.
P.S. The problem is solved by changing <ProjectOnTerrain> from true to false in the model files.