HI,
We probably need to check a few places.
1) Check in System Preferences > iCloud that the name is listed and appears to log in to iCloud.
2) In System Preferences > Mail, Contacts and Calendars check that the same iCloud ID has Messages enabled.
Repeat for all IDs that should be in there that iChat can access.
3) Keychain
In Applications/Utilities open Keychain Access.
In the top left panel select the Login item if not selected
In the lower left select Passwords.
Find and check the items on the right.
Double clicking an item brings it up in it's own window.
Click the Show Password Box (Confirm your Login Password and "Allow Once")
Does this password seem to be correct ?
In the individual panel click the Access tab/Button
Is this allowing Messages/iChat to access the password ?
At present you will not have lost your Buddy list as these are held on the servers involved (Screen Names only) and iChat links with the Address Book for "Real Names".
This will mean that we can delete the com.apple.ichat.(xxx).plist items if required.
7:37 PM Friday; February 24, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously