@johnap987,
Try this ....
Its a bit of experiment so no guarantees ... but its something i tried on a friends system a few weeks backs and it worked for him ..
Logout of iMessage on all your Apple devices and Hacks, Reset iMessage config & security files on your Hack using the method detailed in
Chapter 3.3 ... make sure you do this off-line and ensure that all config files are flushed from the trash by booting in single user mode and following the instructions to clear the trash (important).
Do not sign into any Apple services on your Hack ... just yet, but do login into all Apple services on your other Apple devices.... Wait 24 hours then login in on hack, this should force a new security token to be generated ...
Good Luck
Cheers
Jay