Yesterday night the mysql connection failed because of a configuration issue. That caused worldguard to stop working. I fixed the config (that happened because of the vanilla setup on the new server), but because worldguard failed, fire spread was and remained on. So whenever someone came near a location, any burnable items in the are would spread fire nearby and stuff would burn down.
So I rolled back right now from the last backup 6 hours ago. Firespread was on then already, but all places where people have not been in the last 6 hours are fine again. So sorry for people who lost stuff, but it should be only small stuff.