My iMessage suddenly stopped working in the Messages app and I couldn’t get it to log in properly. In the accounts section it would spin for a while when I typed my password and then give this error:
The registering device does not have appropriate credentials
I finally decided it must be some background task that was hosed and so I poked around and discovered “imagent”:
Killing that processes caused launchd to re-launch it and after that I could log in again. So if you’re having those symptoms, just do this in Terminal:
If you don’t want to do that, then just reboot.