Knowing that, you knew it must be the Mac Pro, fair enough.
Listed at the bottom of this post are the four plists on your Mac Pro that contain settings associated with Messages/iMessage. Repeat the following for each of them (the plists are in /Users/<your user name>/Library/Preferences/):
- Close Messages and move a plist to the desktop.
- Then open Messages again - the plist will be recreated using default settings.
- You may need to log out and back in again
- Un-check and re-check Messages for any Internet Accounts (System Preferences) you want active.
- Test if syncing is occuring before moving onto the next .plist
com.apple.imservice.ids.iMessage.plist
This .plist holds the Apple ID and any verified Aliases (the iPhone Number and any additional emails) linked to the Account.
com.apple.imessage.bag.plist holds the servers that iMessages account should be logging in to.
com.apple.ichat.plist holds most things set in the Messages Preferences (except the Accounts)
It also holds some of the menu settings.
com.apple.imagent.plist
This one for the Saved Status Messages you have made and another that deals with the Recent Items list: