IMessage: Server encountered an error processing registration

Question

Just had my MacBook Pro repaired.  It's almost a year old.  They had to reinstall OS and wiped all my info.  Now no matter what I do, I cannot log into my iMessage. 
I keep getting the message, "Could  not sign in to iMessage. The server encountered an error processing registration.  Please try again later." 
I'm positive that I am using the right password since it works on my other devices. 
Any suggesions?

Answer 1

This could be a complicated problem to solve, as there are many possible causes for it. Test after taking each of the following steps that you haven't already tried. Back up all data before making any changes.
Before proceeding, test on another network, if possible. That could be a public Wi-Fi hotspot, if your computer is portable, or a cellular network if you have a mobile device that can share its Internet connection. If you find that iMessage works on the other network, the problem is in your network or at your ISP, not in your computer.
Step 1
Check the status of the service. If the service is down, wait tor it to come back up. There may be a localized outage, even if the status indicator is green.
Step 2
Restart your broadband device and your router, if different. You may have to skip this step if you don't control those devices.
Step 3
From the menu bar, select
 ▹ About This Mac
Below the "OS X" legend in the window that opens, the OS version appears. Click the version line twice to display the serial number. If the number is missing or obviously invalid, take the machine to an Apple Store or other authorized service center to have the problem corrected.
Step 4
Back up all data, then take the steps suggested in this support article.
There are five steps in that article. Please take all of them. If you don't understand some of the steps or can't carry them out, ask for guidance.
Step 5
From the menu bar, select
 ▹ System Preferences ▹ Network
If the preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Then click the Advanced button and select the Proxies tab. If any proxy options are selected, make a note of them and then deselect them. You don’t need to change the bypass or FTP settings. Click OK and then Apply. Test. The result may be that you can't connect to any web server. Restore the previous settings if that happens.
Step 6
Reset the NVRAM.
Step 7
Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”
While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.
Test while logged in as Guest. After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.
*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.
If iMessage worked in the guest account, stop here and post your results.
Step 8
Boot in safe mode and log in. Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to boot and run than normal, and some things won’t work at all, including Wi-Fi on certain iMacs.  The next normal boot may also be somewhat slow.
The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.
Test while in safe mode. After testing, reboot as usual (i.e., not in safe mode) and test again.
If iMessage worked in safe mode, but didn't work when you booted out of safe mode, stop here and post your results.
Step 9
According to one anecdotal report, Flash Player may interfere with iMessage, though I don't know how and can't confirm. To test this theory, select from the menu bar
 ▹ System Preferences… ▹ Flash Player ▹ Storage
and click
Block all sites from storing information on this computer
Close the preference pane. If this action solves the problem, please post your results.
Step 10
Reinstall OS X.
Step 11
If none of the above steps resolves the issue, make a "Genius" appointment at an Apple Store, or contact Apple Support.