If I had to take a guess at it I would say your using a Anonymous proxy that doesn't send out all of the info that the website needs. Maybe for a security purpose. Therefore the Website does not know who you actually are and cannot let you log in.

Try that one site www.freepublicproxies.com, or google on public proxies. See if swithcing to different type of proxy makes a difference.

Hope I got it right.