My daughter-in-law had this issue and solved it by ‘signing out’ of all instances of Spotify in her online account profile. Then she removed the service from Sonos and the Spotify App from her mobile device, then put things back, signed in and everything worked fine after that, but not sure which of these things actually solved the issue for her.

Thanks Ken_Griffiths, removing and adding Spotify account on Sonos seems to have fixed it.  I should’ve tried that…failing which turn it off and on again