Messages app on Mac closes after login and doesn't receive new messages

This morning, Messages app 26.0 on my Mac with Tahoe 26.7.1, asked me to log on. I did, and it immediately closed. I opened it again and had to log on again. Same thing. I shutdown and rebooted, same thing. I'm not getting alerts for new messages, and no new messages show up for the brief time the app is open. All previous message and contacts are still there.