I read this on another post in the glitches part of the forum and it worked for me just 2 days ago. CLick on someones personal message button (the little bubble under location on the lefthand side of my post for instance. Then on the left hand side there will be a menu, and I believe it says prune messages, click that..and then something will come up which says prune all messages X # of days old or older, and click OK. You should then be OK to view your messages. I did read as well, you lose the messages withing the said time frame on the pop up.
If this is unclear (I am often unclear)then check glitches & forum help-- one of the last options in the boards menu, and on page 1 or 2 there is the post about the same thing....good luck hth