I saw a similar problem posted several months back, but it appears to be a bit different than this one.
Using Firefox 3.6.6 I don't have the following problem. It is appearing on IE 8.0.7600.16385 on my 7.5.3 UBB.threads system as well as happening here on this forum.
After a successful login to UBB.threads, the very next forum page will display a not logged in message. This just started happening and I don't remember changing anything.
My forums are at http://www.dennyp.com/ubbthreads/ubbthreads.php/
but it is also happening on this board.
The other odd thing is that I have a second computer running IE 8.0.7600.16385 also and it is NOT having this issue.
It appears that the problem my be in an option in the IE running on the first computer ... but what option? I didn't change anything manually in IE.
(I have rebooted the first PC a couple of times with no effect.)