I cannot use iMessage on my Mac

Do a backup, preferably 2 backups on 2 separate drives.

Quit the application.

Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library. Then go to Preferences/com.apple.iChat.plist. Move the .plist to your desktop.

Restart the computer, open the application and test. If it works okay, delete the plist from the desktop.

If the application is the same, return the .plist to where you got it from, overwriting the newer one.

If that doesn’t work, repeat the above but delete the com.apple.imagent.plist

Thanks to leonie and Ralph Johns (UK)for some information contained in this.

Hi,

Try this How to Reset NVRAM on your Mac - Apple Support

Try Safe Mode

In the Apple icon Menu top left choose "About this Mac"

This should show you the Serial Number.

Is it being shown ?

If none of of those things work then contact Apple Support.

You will need to state you have an Apple ID issue (free to fix) as they are looking for things to fix under Apple Care that are either Hardware or Software related.

Most likely you will have to ask to speak to a Level 2 person.

They are still likely to have you jumping through several hoops and possibly be in contact with the Engineers at the same time to try and solve this.

Realistically Apple "know" there is an issue where this is triggered but it seems they have not worked out why yet.

The upshot that most people tend to report is that they are asked to wait overnight when they will get a call back but the Login works at some point the next day without the follow-up phone call.

9:54 pm Sunday; November 22, 2015