Try removing YouTube Music from your music services in the Sonos app and re-adding it. Also reboot your phone.

This does not work…..

I have same issue… YouTube Music not working at all anymore.

I removed and reauthorized my YTM premium account and now all the songs are greyed out and “cannot be accessed”.

Can I suggest that you submit a system diagnostic within 10 minutes of experiencing this problem, 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.

As a precursor to calling check your router settings to ensure YT is not blocked. 

I was making the perhaps incorrect assumption that since it was working before, it wasn’t being blocked by the router. But then again, routers tend to update themselves, not always to good effect.