In the Sonos documentation on https://musicpartners.sonos.com/?q=node/224 the following is stated:
If there is a streaming error, the firmware will also show an error in the diagnostic trace file.
How can I see this diagnostic trace file?
The trace file is visible to Sonos employees. You can submit a diagnostic by following the directions here: https://sonos.custhelp.com/app/answers/detail/a_id/142/~/submit-system-diagnostics