1) You are using iChat 5 or Earlier with an iCloud Issued ID from Apple (@me.com or @iCloud.com)
Apple now have the iChat 6 and Messages app log in to me.com as well as AIM and this then Allows AIM to "read" the Password.
Earlier versions of iChat do not do this so you cannot then Login despite the name actually being Valid.
2) AIM servers have a 16 character Limit on Passwords.
You may need to shorten your password. Applies to any version of iChat or Messages.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously