I know how to fix the problem (At least until the Techs can figure out the cause) .
However many New Messages you have in the negative, for example I had -8 New Messages, Send yourself a PM
by clicking on your username, (I just wrote Subject: Hello and the Message: Hello again) then answer it.
After you have answered your own PM, delete the original one, and your Negative count will go down.
Repeat for each Negative New Message.
You can also try *just* clicking "reply" and them backing out before hitting send. Saves the trouble of actually sending yourself (or some other "lucky" member) all those PM's.