I have a Sub Mini experiencing the exact symptoms reported with the Boost. It was connected to a Beam 1 and 2 Play:1s and working perfectly until yesterday evening. At point of update last night the Beam and Sub Mini were both failing to update to 15.1. I disconnected the devices from each other and the Beam then updated. The Sub Mini is now completely disconnected and shows as ‘unused’ in my devices list. It says it needs to be updated.
It won’t complete the update, failing at the ‘Finishing Up’ stage with the update problem and try again message after taking a long time and therefore, because all of the other devices are on 15.1 it won’t join the system and so, my expensive subwoofer now is unusable.
When the update does error out, it does also have the blue dot with ‘updating’ next to the Sub Mini in the System Menu but obviously, it never works. I force close the Sonos app and it asks to update again. And yes, I’ve tried rebooting routers, logging out, force close and everything suggested on the Boost thread.