Sorry for the triple post, but there's yet another different problem with these. It's the joined rooms again:
Joined rooms cause some of the problems and work in strange ways overall, resizing or moving them helps sometimes.
I've got rid of all the problems and my cell now seems to render everything perfectly (except for water but it's just too tough to fix), but there's an issue with one spot, where two room markers are joined. Moving into the area where the markers overlap and facing a certain angle makes the whole screen go black. Not like seeing into the void or the objects disappearing, just pitch black. The direction needs to be pretty much exact for this to happen, I think it's about 90 degrees to the left (the area is in a straight hallway). I also had the same problem in the middle of another room when I added multiple occlusion planes inside 2-sided walls. Moving close to the wall in a certain angle made the screen go black.
Anyway, the joined rooms are the thing I'm confused about. I've tried resizing, rotating and remaking them, but they always have the problem. They sometimes give an error when loading the cell in CK or pressing F5, but not always. I've had similar problems with other joined roombounds (though I never got a black screen), but modifying or remaking the markers has always fixed the issue. However, nothing seems to help with these ones. What am I missing here?