If this is an Apple ID that happens to be valid with AIM you need to keep the password to 16 characters or Less.
If it is an AIM or AIM Valid Name go to iChat Menu > Preferences > Accounts > Server settings and untick the SSL option.
I take it having changed the password as the place the ID or name is registered you have changed it in iChat ?
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
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