The problem, 99% of the time, is a corrupt cookie. Follow these steps:
1. Log Out (even if you are already logged out as this usually deletes pprune cookies) or just go to:
http://www.pprune.org/forums/member....&action=logout
2. Go your browsers settings for cookies and delete any cookies for 'pprune.org' that you find.
3. QUIT/EXIT your browser (Not just the window you were in).
4. Restart your browser.
5. Go to
http://www.pprune.org and log in with your Username and password.
If the above doesn't stop the problem then I'm afraid I can't help you. The problem is invariably a corrupt cookie on your PC.