"Block Guild Invites" Bugged?

Hello.
I can't seem to change the "Block guild invites" option in Interface menu. If it was disabled, I can't enable it now and vice-versa. When I change it, click OK and check again, I see the option reverts itself back to the previous state. I've tried this in all my alts, with different WoW.exe files, entirely different clients and the problem seems to persist. Changed entire WoW folder (or whatever portion of it I can) for write access and still no avail.

Any ideas? :|

It is bugged on the Mists of Pandaria realm.