I suspect there’s some sort of handshake issue with the Spotify servers. Not sure where the issue lies, however, so I’d try a couple of things on the Sonos side. First, I’d power cycle the speakers. In fact, while the Sonos speakers are unplugged, I’d also power cycle the router, waiting a couple of minutes for it to come back up before powering back up the Sonos. This will force both the Sonos and your router to reload their respective firmware, and ensure the Sonos get fresh IP addresses from your router.
The challenge is I’m having trouble understanding if the issue is in the Sonos controller, as suggested in your post, or in Spotify Connect, as suggested by your tags. If it’s the former, I’d certainly remove Spotify from your Sonos controller, wait two minutes, then add it back in, which will force a new handshake with Spotify’s servers. If the issue is with Spotify Connect, they own that software, and you’d need to move further with their Customer Support.