I'd like to report the result of a troubleshooting session.
My Sonos App stopped on Android.
Refreshing, uninstalling and reinstalling the app and restarting it produced no results.
No attempts at rejoining or setting up the system were successful.
The speaker system was however perfectly accessible and functional from different devices and from the desktop MacOS app.
Once I removed the zScaler VPN app from the Android phone, the Sonos app was able to connect to the system immediately and quickly.