Despite your reboots, which I do think are an effort in the right direction, I can see two potentials here. One is local, with some sort of duplicate IP address issue. You need to ensure the Sonos devices are unplugged from power when you reboot the router, otherwise they’ll store the IP address and try to reconnect to the same one when the router comes up, and depending on which IP is ‘duplicated’ it’s possible it won’t clear the issue properly.
The other, of course, is the possibility of a issue with the Spotify server itself, or, I suppose, the path outside your local network to the Spotify servers. I see this as a fairly low possibility.
I’d try the reboot sequence as described in my first paragraph. In the unlikely occurrence it doesn’t resolve the issue, I’d cause the problem to happen again, and then submit a system diagnostic and call Sonos Support to discuss it.
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, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.
