I've had this problem before. Here's what happened to cause it:
I would login as the admin on a site, then logout to log back in as a different user. I make my posts, then go back to the login screen (without logging out the regular user) and log back in as admin. It seems to work fine, until I go to the admin section. Then it says I have to login and takes me to the login screen (but at the login screen it recognizes me and just shows me my PM's).
To fix, I just deleted my cookie for that forum, logged back in, and remembered to always logout before logging in as a different user. BTW the forum name was something like
www.domain.com/forum, and the cookie name was something like forum[1]. Hope that helps.
