I just upgraded to 2021.9.0 tonight and after I did that I noticed my Sonos devices seemed to be popping in an out. I’ve never noticed them to be offline for over a year and suddenly it looks like they’re being intermittent:
I’m hoping it’s not my network but I guess it could be. Anyone else experiencing this with 2021.9.0? I have a VLAN setup so I had to find and allow all the ports that Sonos needs but that has been working up until today.
Any messages related to Sonos in your logs? If you have issues with subscriptions arriving it will print a warning message.
Thanks @jjlawren. I just figured it out. It was something I did to my network to enable a new piece of equipment… I stupidly ignored one of the critical rules of networking – don’t assume that a change won’t impact other stuff. 
At least I got it back online but the new equipment is going to have to be returned.
If you have questions about the Sonos networking side of things, feel free to post an update.