I have the same or similar problem with logging in at gamesquad. What works for me is
A) if I want to post to the forum, I log in while on the forum page I want to reply/post to
B) if I want to download a scenario or other, I click on the header tab, get the "You are not logged in" message, log in there, and then get to the scenario/map/other page.
I cannot go back from download to forum, nor can I post and then just go to the d/l page. I have read others complaining of this, so while this could be all my own mistakes, I think maybe not.
And, for some reason, I often type in user_name in the password box and password in the user_name box -- it's just a confusing set up to me. Anyway, hope that either my procedure and/or knowing you're not the only one with a problem is some help.