I've changed the setting back to be cookie based. If you've cleared your cookies, all threads and forums may look read, but it is exactly back to the way it used to be now.
Let me know if this solves your invisible setting too. This is the only setting that was changed in the last 24 hours and it doesn't make sense to me how that disappeared from your control panel... but if this solves it, then I suppose we are good to go again! :)