I thought maybe it was a problem with Firefox, so I tried using Internet Explorer, but it had the same errors. Cleared passwords, cache, etc...still happens. Using the little "new" icons on the thread subject doesn't take me to the latest post in the thread anymore either. It just sends me to page 1 of the thread.
Another thing I noticed is that my msg count now says "You have 0 messages, 0 are new." instead of "You have -1 messages" which occurred when I deleted that spam PM we all got awhile back.