Internal Mail Database Error (Need a Blue)

I posted this issue in Technical Support several hours ago, however I was just advised that this might be the forum that my problem belongs in.

Around 4am EST, I noticed that I lost the ability to collect items from my mailbox in game. Any attempt to collect gold or items gives the message: Internal Mail Database Error.

Additionally, I am suddenly unable to log out or exit the game.

I have deleted my WTF, Cache and Add-Ons (every single one). I have rebooted my computer several times since.

Strangely, this problem is only affecting one of my accounts; my other account which is primarily Alliance and linked to the same battlenet does not experience any of these issues. I have tested one, then the other, then both simultaneously via dual-boxing. Only my Horde account has the problem.

For reference, this person had the exact same problem:
http://forums.worldofwarcraft.com/thread.html?topicId=24401792504&sid=1
Therefore, it is not related to the last patch.

Upon further testing, I see that I cannot list items on the ah or use items or skills in any way; an attempt to hearth or cast Power Word: Shield, for instance, gives me the error "Another Action Is In Progress".