Hello,
This occurred on my old vBulletin installation and my new Joomla installation, so I'm fairly sure it's a serverwide problem. The issue is that certain users, even after accepting all cookies, clearing them, and refreshing... Just cannot get logged into the site. Two of the users with issues both say they're using MSN, if that helps. One has parental controls turned on, the other does not.
Sometimes it will log them in, but if they click a link, they will immediately be logged out once again.
I appreciate any help or direction you can turn me for this problem!