Why does Deezer work so badly on the Sonos app 95% of the time?

Apologies, there is no screenshot. So I can’t see what error is being generated. Have you tried removing Deezer from Sonos, waiting five minutes, then adding it again? Have you tried a simple reboot of both your Sonos and your router? Have you called Sonos Support to discuss it?

When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Oops, this time it loaded/showed the screencap, let me review. 

That sure looks like a failure to reach the Deezer server to get any information to populate the data. I’d really be tempted to do a network refresh , by powering off (unplugging) all Sonos devices, then rebooting the router. Wait two minutes, then plug back in the Sonos. Wait another two minutes before testing. It also may be worth reviewing the wifi interference FAQ, it could be the Arc is being blocked in communicating with your router. 

But if neither of those help, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. Don’t post the resulting diagnostic number here.

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 Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

The strangest thing is that Spotify works perfectly and with enviable speed.

Could easily be a DNS issue between the speaker and the Deezer router. My suggested reboot sequence might solve that. 

Not just the Deezer server, many services require connection to other servers, not in the deezer domain, to work. 

Sometimes an update to anti-spam or tracking services can block something essential to proper functionality.

True, but DNS errors can affect one server connection, but not necessarily another. But I do agree, all data that shows in a Sonos Client from a service originates with that companies’ server.
 

It certainly could be another app blocking (incorrectly) access, though, which is why I used the word ‘might’. 

I can just add that it’s not a general problem. From DE Sonos reaches Deezer servers without any issues and works like expected.