Messages Doesn't Communicate with AIM Server - Can't add/delete buddies, change privacy settings

Question

This has frustrated me and I think I've diagnosed the problem but I need help!
The problem occurs in Messages for Mountain Lion. I've set up my AIM buddy list and everything seems like it's working. But, when I try to add a buddy, it doesn't matter whether they are online or not, they appear to be offline. Then, when I quit Messages and launch it again, that entry has disappeared from the buddy list altogether. The same thing occurs when I try to delete a buddy or anything else that involves a change at the AIM server.
The problem is the Contacts app. I know that. If I trash my address book, everything works perfectly! If I add cards back one by one, the card that breaks Messages is always a different one. There isn't one "culprit." I also know that it's probably because I have had these contacts for years and I have migrated them from platform to platform. It's not a local problem though: if I trash my address book and then sync with iCloud to get my contacts back, it happens again.
There's something about the way Contacts and Messages handle AIM that doesn't allow changes to propagate to/from the AIM server and I can't discern any pattern that would lead to a solution.
Does anyone have any insight here? Is this just a bug in Messages?

Answer 1

Note: These instructions are for OS X 10.8 or later. In OS X 10.7, the procedure is the same, but the application is named "Address Book" instead of "Contacts."
Step 1
Back up all data.
Triple-click anywhere in the line below on this page to select it:
~/Library/Caches/com.apple.AddressBookSourceSync
Right-click or control-click the highlighted line and select 
Services ▹ Reveal
from the contextual menu.* A folder should open with an item selected. Move the selected item to the Trash.
Log out, log back in, and test. If there's no change, go to the next step.
*If you don't see the contextual menu item, copy the selected text to the Clipboard (command-C). In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.
Step 2
A. If you use iCloud, uncheck the box marked Contacts in the iCloud preference pane. Press return to confirm, or click  Keep on Mac in the dialog that opens.
B. From the list of groups on the left side of the Contacts window, select All On My Mac. If that group doesn't exist or is empty, skip this step. Otherwise, from the menu bar, select
File ▹ Export ▹ Export vCard
Save the exported contacts to the Desktop.
C. Quit Contacts. Triple-click the text on the line below on this page to select it, then copy it to the Clipboard (command-C):
~/Library/Application Support/AddressBook
In the Finder, select
Go ▹ Go to Folder
from the menu bar, paste into the box that opens (command-V), and press return.
A Finder window will open with a folder selected. Move the selected folder to the Desktop, leaving the window open for now.
D. Launch Contacts. Your contacts will be gone. If you took Step 2, then double-click the file you created in that step and confirm that you want to import from it. Your contacts will reappear.
E. Re-enable Contacts in the iCloud preference pane, if you disabled it. Test. If the issue is resolved, delete the AddressBook folder and the vCard file on the Desktop and close the open folder window.
Otherwise, quit Contacts again and put the AddressBook folder back where it was, overwriting the newer one that will have been created in its place. Post your results.