I'm going to take a stab at this; maybe I'm on to something, maybe I'm not. There are potentially 3 different account servers that that C2 is trying to authenticate with. (there could be even more; these are just the ones I know about).
gamesas.com
ea.com
gamespy.com
You may think that you're authenticating on your gamesas account but in reality it's matching on a different account and if you have different passwords then that could be why you're having authentication issues. (does this sound familiar? Remember people having problems redeeming codes and having the website tell them they aren't logged in even though they are? Same issue)
There's two things you can try.
1 - Make sure all applicable passwords are identical.
2 - Change the email addresses on ea.com and/or gamespy.com as applicable.
Then try logging in with your gamesas credentials.
Then blame EA for this crap.