After uninstalling Avast Free totally, I installed newer beta, 22.10.6038 (build 22.10.7633.734).
This problem has gone away but returned when I restored my settings backed up earlier. So this problem is caused by restoring settings backed up by previous beta. Maybe because newer version has new or changed settings which is incompatible with the settings backup earlier.
After I 'reset all settings to default' in Troubleshooting and rebooted, the problem went away again. Of course, I had to change my settings manually all over again.