Greg,

I'm not sure what's causing this. Could you try a few things?

1) It's possibly being caused by a message from a banned member. If you have any of those delete them. If not try deleting all your current messages. If you are saving your sent PM's try clearing those too.

If you can't delete them because of the error let me know as I can mass delete a users PM's.

2) Could also be a login/cookie issue. Firstly when you login make sure "remember me" is selected. If it already is try logging in from a different browser (ie Firefox or Chrome). Or try clearing your cache/cookies/browser history in your current browser.

Let me know if any of that works or not.