Messages wedged, identityservicesd, callservicesd repeatedly crashing
Hello. I can neither send nor receive messages with Messages.app on this system. Recently downgraded to Sierra to fix a video isssue with High Sierra. I restored user data with Migration Assistant, now Messages is wedged/borked/broken. Please don't suggest reinstalling Sierra, I already tried that.
Symptoms: Messages not working. Cannot access Messages prefs or Buddies menu. Preferences in Messages menu are grayed out:
Attempting to access the Buddies menu creates a spinning color wheel that will not die. Force quit is the only path out of that hole.
Identityservicesd and callservicesd are repeatedly crashing. /var/log/system.log is showing these repeated messages.
Oct 7 10:05:40 minimus com.apple.xpc.launchd[1] (com.apple.identityservicesd): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
Oct 7 10:05:42 minimus com.apple.xpc.launchd[1] (com.apple.telephonyutilities.callservicesd): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
I also see corresponding .crash files in . ~/Library/Logs/DiagnosticReports.
So, the question in my mind is, what files do I need to /bin/rm to reset the Messages framework to some default initial state so I can reenter my message account login details (e.g. iCloud)? I am an old unix geek.
Mac mini, macOS Sierra (10.12.6)