My daughter in law had a similar issue with her Spotify account on Sonos and with my help, what she did was the following…
- Logged into Spotify online account and then selected the option to log out of Spotify everywhere.
- Removed the Spotify App from Sonos
- Set her routers 2.4Ghz WiFi band to use a fixed non-overlapping channel 1, 6 or 11 and a channel-width of 20Mhz only and rebooted the router.
- Powered off all Sonos speakers and brought them all back online and then reinstalled the Spotify service via the Sonos App.
Spotify then all worked for her perfectly after doing these things and she has not had an issue since. If she groups her speakers, she now mostly starts that group-playback using the most modern Sonos speaker in her setup, which in her case, is a Sonos Five - So that new(ish) device is usually the group co-ordinator - it has a decent processor/storage for sending all the music to the other devices for playback in sync.
So perhaps see if these things may work for you.
thank you, this is something good for the coming snow! Much appreciated.