Did your iMessage on your mac suddenly stop working with an error message of “The registering device does not have appropriate credentials.” After some searching we found the solution from our friends at https://porkrind.org/missives/imessage-on-mac-os-x-not-working/
There are two ways to fix this:
1. Open Terminal and enter killall imagent and then try to use iMessages again. That should fix it right away.
2. If you don’t know how to use Terminal, try restarting your computer (the old magical way of fixing all computer problems).
Hope this helps many of you as it helped us!