I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it. But don’t post the resulting diagnostic number here, they get sensitive about GDPR.

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.
 

Just as a side note, there does seem to be an issue with Mixcloud’s servers at the moment, but I’m unsure as to whether it is S1 or S2, or both, and if it would impact your Sub. The fact that it works on Spotify but not on Mixcloud suggests it isn’t a Sonos issue per se, but an issue with the conent being streamed to you by Mixcloud.