I'm making a simple cuboidal room, but as soon as I solidify my mesh it gets weird black patches around the corners.
If I turn fill rim to off it looks good, but obviously I want to fill the rims. I guess the normals are flipped or something leading to a lighting issue. It also persists in final rendered version
From outside
Notice the dark shade just above the 3D cursor

From inside
Again above the 3d cursor

File.