I'm running Big Sur (11.0.1) on a Late 2014 Mac Mini, and the Messages app refuses to stay logged in.
I recently created a child account for my eldest. Signed into her account on Mac Mini, I have successfully signed into her account in
Apple ID in
System Preferences, and into
When I open the Messages app, it prompts for credentials. I enter her creds (that I just created), and all seems to go well. I tried sending a message to my account, which was never delivered. Then, within a matter of seconds, the app appears to either restart or decides that authentication needs to be re-performed, almost like her account is being logged out. This repeats no matter how many times I try, or whether I restart the machine.
If I log into my Mac account, open Messages and log in with my Apple ID, the same thing happens. When I just tried, it lasted seven seconds before restarting.
If I open Messages using my Apple ID on a 2018 Macbook Pro, also running Big Sur, everything is fine and I can send messages to friends with iPhones, using their phone number. Similarly if I log in as her account on my MBP, I can send messages.
If I try to send a message to my child's account using my MBP, I get "Not delivered" errors.