I duplicated my level. Started deleting massive chunks of it until I started to get close. Then I deleted one at a time until I was left with 3 brushes. Deleting any one of the 3 fixes the issue. Two of the affected brushes were a part of a decoration so I converted the whole decoration to a static mesh. The new static mesh gave me the same error, but with the mesh name instead of none. I recreated the static mesh at the origin and finally the issue is solved.