Nekro, give this a try if you haven't already.
I couldn't log into GFWL Marketplace at home, only at work, due to some login error (random code 848578 or something). I had my 'remember me' box checked so it was trying to log me in automatically and it failed, despite being the correct info. Unchecking those boxes and logging in manually let me in.
Maybe give that a try? It probably won't help, but who knows with this garbage