Messages won't activate after 10.9.3 Update

Question

Hi,
I updated OSX on my late-2013 Macbook Pro Retina 15" last week. Since then I can not log into Messages.
I get an error message - "Could not sign in to iMessage. An error occurred during activation. Try again.".
I've tried many times. I've checked what I can see in the knowledge bases -
Time/Date is OK, timezone etc. OK.
Serial number is correct on system information. Checks out OK in apple support site to check.
No network issues (and tried in several networks)
Apple account OK and works on my other Macbook Pro, iPhone and iPad
I've tried reinstalling the combo 10.9.3 - no change. Repaired permissions. 
It's very frustrating - has worked fine up until this point.
Any ideas? Not the kind of issue I was expecting after an update with current Mac hardware etc.

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
Sign out of iMessage on all your Apple devices. Log out and log back in. Try again to sign in.
Step 3
Restart your router and your broadband device, if they're separate. You may have to skip this step if you don't control those devices.
Step 4
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 invalid according to this web form, take the machine to an Apple Store or other authorized service center to have the problem corrected.
Step 5
Take the steps suggested in this support article. If you don't understand some of the steps or can't carry them out, ask for guidance.
Step 6
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 the box marked SOCKS Proxy is checked, uncheck it. You don’t need to change any other settings in the window. Click OK and then Apply. Test.
The result may be that you can't connect to the Internet at all. Revert the change if that happens, or if iMessage still doesn't work. Remember that you must Apply any changes you make in the preference pane before they take effect.
Step 7
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.
Step 8
Make sure you know the ID and password you use with 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. Click Utilities, then Keychain Access in the icon grid.
Use the search box in the toolbar of the Keychain Access window to find and delete all items with "iMessage" or "com.apple.idms" in the name. Log out and log back in.
Step 9
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 10
Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you start up, and again when you log in.
Note: If FileVault is enabled, or if a firmware password is set, or if the startup volume is a Fusion Drive or a software RAID, you can’t do this. Ask for further instructions.
Safe mode is much slower to start and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal startup 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, restart as usual (i.e., not in safe mode) and test again.
If iMessage worked in safe mode, but still doesn't work when you restart in "normal" mode, stop here and post your results.
Step 11
Reset the NVRAM.
Step 12
Reinstall OS X.
Step 13
If none of the above steps resolves the issue, make a "Genius" appointment at an Apple Store, or contact Apple Support.