Are you signing onto the forum in its own window, or in a window off the main site?
I've tried both to no avail. It still says that my username is not found. Since I can log in at school I know my ID works. I even created a second ID at home at that worked, logged out and then tried to log on with my old Id and still says username not found.
Lowering the security settings in IE to allow all cookies fixed it for me.
I lowered the security setting to lowest setting to allow all cookies and it still doesn't work