Messages on Yosemite quit syncing

Question

Hello,
I have been using Messages on OSX for a while now, but all of a sudden yesterday it quit working.
I will skip through a bunch of troubleshooting. I found that I can create a new user account and Messages works fine in the new account. Therefore, I would like to delete any preferences, caches, or other settings that lets Messages know it has ever been run before and start fresh. Can anyone tell me what I need to delete? I went in to ~/Library and deleted all caches and preferences that seemed relevant, but when the app comes up it still has old conversations in it and still won't recognize numbers that I know are associated with Messages from other people.
Any suggestions?
Thanks!

Answer 1

Back up all data.
Quit FaceTime and Messages if they're running. Force quit if necessary. After each of the following steps, restart the computer, then test. If the problem isn't resolved, quit again and go on to the next step.
Step 1
Make sure you know the ID and password you use with FaceTime and iMessage.
Launch the Keychain Access application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Select the login keychain from the list on the left side of the Keychain Access window. If your default keychain has a different name, select that.
If the lock icon in the top left corner of the window shows that the keychain is locked, click to unlock it. You'll be prompted for the keychain password, which is the same as your login password, unless you've changed it.
From the menu bar, select
          Keychain Access ▹ Keychain First Aid
from the menu bar and repair the keychain.
From the Category list in the lower left corner of the window, select
           My Certificates
Look carefully at the list of certificates in the right side of the window. If any of them is marked with a red "X" as expired or invalid, delete it.
Now select
          Keychain Access ▹ Preferences... ▹ Certificates
There are three menus in the window. Change the selection in the top two to Best attempt, and in the bottom one to  CRL.
Next, select
            Passwords
in the Category list. Enter "ids" (without the quotes) in the search box in the toolbar of the window. Delete all items of type "application password" that have a name beginning in any of the following ways:
            ids:
            IDS:
            com.apple.ids:
Also find and delete all items with "facetime" or "com.apple.idms" in the name.
Step 2
Hold down the option key and select
          Go ▹ Library
from the Finder menu bar. Move the following items from the Library folder to the Trash (some may not exist):
          Application Support/CallHistoryDB
          Caches/com.apple.imfoundation.IMRemoteURLConnectionAgent
          Caches/com.apple.Messages
          Containers/com.apple.AddressBook.FaceTimeService
          Containers/com.apple.corerecents.recentsd
          Containers/com.apple.FaceTime
          Containers/com.apple.soagent
          IdentityServices
Leave the Library folder open.
Step 3
In the Preferences subfolder, there may be several files having names that begin with one of the following strings:
          com.apple.facetime
          com.apple.FaceTime
          com.apple.ids
          com.apple.im
Move them all to the Trash.
Also in the Preferences folder, there is a subfolder named "ByHost". Open it and do the same thing.