About 2 months ago one of my Sonos speakers started acting erratically, disconnecting frequently, stopping after one song, throwing error messages, etc. The usual solutions did not work, or worked only briefly. Finally, a call to Sonos revealed that the speaker was trying to connect to a wifi network from a previous address about 5 years ago. Why this didn't cause problems until recently, I have no idea, but deleting this network from the list solved the problem instantly.

I'm posting this in the event it helps someone, as I've not seen it explicitly raised elsewhere.