Does your client use Sonos?
Is it possible someone is restarting the stream on one of the Sonos when your client isn’t at one location?
If a Sonos is involved, it would only show that ‘error’ when instructed by the Pandora authorization server, and not by a Sonos server. But I’d still recommend that your client submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. They, or you, shouldn’t post the resulting diagnostic number here, Sonos’ lawyers get sensitive about GDPR, I suspect.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When they, or 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.