My iMessage suddenly stopped working in the Message app on Mac OSX Mountain Lion and whenever I tried to re-login I kept getting the error: Code: The registering device does not have appropriate credentials To fix this, all you have to do is open a Terminal Window and Type: Code: killall imagent Job done! Restart iMessage and re-login with your Apple ID and it will be good to go again! Reason: It appears the background task imagent got stuck, and by killing it you force Mac OSX to re-launch it.