Most everytime a person has log in problems is that they are logging in through battlenetwork, or mewgood, or something else.
I'm finding that there are sites out there that are linking to us as the forum for their site.
We're getting the business so I don't see the harm.
But sometimes when they do this they add stuff to the url that can screw up the login as well.
www.battleforums.com is the best advice to give.
Those few that still have problems after this is usually due to browser settings, or cookie issues.