Big Sur - Messages not starting
Broke for me after the update to Big Sur 11.4. Tried every fix in this thread to no avail, about to jump on Apple Support. My message icon shows an incoming text, but when I try and open Messages on my Macbook Pro it ***** out with "Messages Quit Unexpectedly."
I was getting same issue and was on chat with an apple technical support executive who gave me this solution. Try resetting your password for Apple ID and also reconnect or update your apple 2FA devices.
https://support.apple.com/en-gb/HT204915
The token to authenticate and give access to these services with Apple get expired and reseting password or refreshing the device for 2FA clears / refreshes it.
It took me less than 10 minutes to do all the points to solve this with support team. Hope this helps resolve the login, auth issue.