This morning I opened the forum and got a pop up alert of a PM. I clicked on it to open the message and read it, but it still shows one unread message. 
Now every time I click on the "General Topics" box that gives me the starting page with all sections I get the same pop up telling me I have a message, even though I've got no new messages. 
I've tried refreshing the page, leaving the site and coming back, shutting down my computer and restarting. But nothing makes the site acknowledge I've read all messages, and stop sending me the alert that tells me I've got a new one!
What do I do to stop this annoying pop up when I have no new messages?