There's a bug in the mice/mouse kingdom that makes for a certain amount of time the whole floor of the kingdom pitch black.
I believe that's related to a memory leak issue, when the game runs for "too long".
We're currently optimizing everything up and see if that can be prevented that way, in the meantime I'd suggest you save and restart if that happens (it never happened to me, but I've read reports saying that is supposed to fix it).
I don't envy you guys trying to figure out what's causing it and find a fix for it, memory leaks can be so damn hard to identify unless you know exactly what's causing them. Even having a general idea can involve a dozen wild goose chases, I'll never forget the many hours I worked on one trying to work out how hair textures were causing it when the whole time it was a quest item causing a sorting bug in the inventory that was invisible to the player.