I have this error in my inbox, and my little statusbar in the forum tells me I have 1 unread pm constantly :( .
It shows a pm with no title from no sender....I got this after I got a notice about a new pm, but my pm box was full, so I emptied it accordingly and got this :o .

I have attached the image below :) :

I ran a repair and optimize for the PM tables, still showing up?

Yes, for some reason it still is :( .

Had the same thing a couple of days ago......just deleted it when I realized it was just a glitch. It hasn't returned.

There, I got rid of it; but I guess that's a glitch like you said with vbulletin.