Messages restarts to login screen after a few seconds
What i can recommend for you is to download “iMessage debug” Its a terminal Applcation,just unzip it and double click it to run in the terminal. You will have to goto system preferences and click on the security and privacy icon and allow it to run because it will blocked because of the “unidentified developer crap” But anyhow just run that and see if that fixes your problem. If there was a known good imessage configuration on the said Mac at one point then this app will be able to restore that configuration automatically. Your ethernet in your Hardware config will have to read as “en0” and “built in”. And your nvram must be working as well. In terminal you would type the command “sudo -s” enter your password and then “sudo nvram -p” hit enter. “sudo nvram -c” now type “sudo nvram mymac=1234” where 1234 can be whatever you want to put there and then just hit enter. Then reboot and then go back to terminal and just type “sudo nvram -p” and it should bring up some data on the screen along with whatever you named the 1234 above and that way you will know if your nvram is working. If it doesnt show the 1234 rename then its not working and that would be the problem. Anyway hope this helps