Not enough information provided to begin to guess. Have you tried a call Sonos Support directly to discuss it?

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 network and Sonos system.

You’re right. Not enough information. Last night, it was below “Favorites / Pinned Collection.” Another night, it was when I tried to search Apple Music. Seems to pop up when I start listening late at night. Seems to be fine this morning. And, no, I haven’t tried calling Sonos Support. Tends to be there as I get ready for bed. Not a convenient time to try to call Sonos Support. 

That’s helpful. And I understand the timing issue. You could always submit a system diagnostic, and call later. 
 

The only reason I can think of for it to ‘be there, then not be there’ is some sort of undefined communication issue, either between your Sonos devices and your router (and by extension, the internet) or your controller and your router (and by extension, the internet). But that leaves a lot of things to look at, which is one good reason to do the diagnostic, and call in. Hopefully, there would be data suggesting an issue that they could see. 

The alternative is a certain amount of time and effort on your part, at the end of which may still be a diagnostic, and phone call. 

Bruce, Thank you… Art

In my case it is because the web app is terrible at loading data from music services, due to the grossly inefficient way it works now.

If it works, it’s slow, but when it takes too long (10, 20 seconds?) it times out and gives the above error message.

Probably right… This issue seems to come and go. And, sometimes resolves itself after 10 or 15 minutes. Meanwhile, I’ve  moved on to Apple Music app.