Solution
I fixed the error only by setting all brushes touching the void to structural brushes. [HeavyBlade]
Just ran into this one on a small testmap, a few detailed detail brushes surrounded by a hollowed box of caulk, which was structural and touching one edge of the detail brushes. Got rid of the error by making the caulkhull not touching the detail and getting rid of the overlapping parts of the caulkhull.
