Hi,
Ever since iChat was at version 1 (no video or Audio chats) it has been an AIM client and Apple issued IDs have been accepted as Valid AIM Screen Names.
Back then Apple was using the .Mac Service and you got @mac.com ending names.
Whether you have Trial Account, a paid for one or one that Lapsed they all work still as Valid AIM Names.
Then Apple changed to MobileMe and IDs ended with @me.com.
Trial Versions of these only worked for 60 Days (until you were charged or Actively ended the Trial).
You could link an older .Mac ID to the new @me.com ID for Mail purposes.
They remained two separate AIM valid Names.
More recently Apple have changed to iCloud.
Initially this created more @me.com name but on the iCloud Servers.
Later Apple started to issue @iCloud ending IDs
If you had an MobileMe @me.com ID and have not "converted" it to iCloud it will no longer work as a valid AIM name.
Due to changes in the app at iChat 6 which includes a second login to Apple as well as AIM (which in turn allows AIM to "see" the password) the iCloud issued @me.com and @icloud.com names will not work in iChat 5 or earlier.
This means that in Messages in Mountain Lion and Mavericks you can use your Apple ID to "register" for iMessages AND use it as An AIM account.
Back to your issue.
As a first step go to Messages > Preferences >Accounts
In the AIM account Untick the 'enable this Account" box to Log out.
Now go to the Server Settings tab
The Server name and Port plus the check boxes can now be changed.
Untick the SSL option and then Log back in again.
8:00 pm Wednesday; January 15, 2014
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
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