So does that mean you are using AIM plus @mac.com and @me.com names ?
Do you mean that you have an Apple ID that was @mac.com that was then linked to iCloud that gave you the @me.com version ?
Or do you mean you had and .Mac (@mac.com name) that was paid for and then MobileMe and used both names in a linked account ?
Which name are you actually using in Messages ?
The Password still needs to be 16 characters or less when accessing the AIM server in Messages or iChat.
If you have a linked @email@example.com account then the password will be the same although you can use the Names as separate Screen Names in Messages.
I am having no issues with logging in 2 AIM registered names, an @icloud.com name, an @me.com linked to the same Apple ID and two older @mac.com names.
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