What URL are you accessing the board from? What is your "Full URL to Main Directory" setting (CP -> Master Settings -> DB, Paths & URLs -> Paths & URLs Tab)?
If you're accessing the site from the non-www format of your domain and the above setting is set to the www domain it's possible that you're being logged in on the non-www but being thrown to the www which is not logged in; a fix would be to add an apache .htaccess rule to force one domain or the other (can be provided if needed).
