Likely due to some sort of local wifi interference , although it could also be a duplicate IP address, or any of several other network issues. 

The first thing I would do is read through that FAQ I linked above, and apply any of the solutions there that may be pertinent to your network. 

Next, I’d perform a simple refresh of my local network, by unplugging all the Sonos devices from power. While they are unplugged, reboot the router. Once the router comes back up, plug back in the Sonos devices. 

Those two processes would cover a multitude of potential reasons why you can’t play Spotify due to local issues, but don’t address any external to your network issues, some of which could be indicated by the ‘real time status’ link next to this post. 

However, if it still is continuing, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, or post the diagnostic number here for a Community Moderator to pick up.

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis.