I just looked through the HTML to see what values are or aren't being set. I am just missing two "rmShown" and "GALX". The first one I'm guessing is just a UI thing and shouldn't affect login. The second looks like a token of some kind. This is the only suspicious thing I can notice as for why some can login but not others.

Ooh, I just got another idea but its a long shot since I should be handling this correctly but it is untested. This is pretty personal but do you have any non-alphanumerics in your username or password?