Uh, well, the bug seems to have disappeared, maybe. But the server became daytime in the meantime, so maybe it only happens at night. I'll post again later if I see it...
After I connect to a nighttime server, my game looks like this: http://i.imgur.com/FBnbrKK.png My graphics card is a 7970. It runs other games just fine, no corruption. I'm inclined to think it's some kind of driver or shader bug? Has anyone experienced this before?