unable to play - server cannot be found

That's great, Tim!  Nice to hear that someone's experience has improved.

I use a Sonos bridge and don't use the new wifi mode. I have, however, just made the same sort of router config change to dedicate/reserve the assigned IPs to the various Sonos devices. If that makes a difference in stability of the wireless connectivity of the devices and their access to the music share, awesome. But it would be a mitigation on a regression introduced with the introduction of the bridge-less/wifi-only architecture.

I didn't reserve any IP addresses in my router to fix this; I found that deleting the "Homegroup" settings from my PC took care of it.. I use the bridge and just have DHCP enabled on my wireless router.  When I called tech support he logged on to my PC and couldn't find anything wrong with my setup and said they would have to continue looking into it.. I'm not one for having much patience when it come to my tech not functioning so I started looking at what might have been missed. and that was it.. I hope that this helps someone else.

Having same problem. Submitted diagnostic 4302298 

My firewall is turned off and was working last night without any changes to my system.

Sonos Customer Service helped me figure out that I had a bad cable between my router and bridge.  I would have never guessed as it was essentially brand new. Replaced that and all is good!

Sonos Customer Service helped me figure out that I had a bad cable between my router and bridge.  
Hi guys, I'm a Mac user and I just thought I'd let you know that Sonos customer services were great and helped sort this out. They said that with the last Mac update, it's affected Sonos, renamed the Mac and the new name was too long.

We renamed it in 2 places: WINS in Network Preferences/advanced and also under Sharing. It then connected perfectly and remedied the issue. Hope this helps.

I am experiencing same on a MAC.  SONOS should get something posted on this. 

Sonos Customer Service helped me figure out that I had a bad cable between my router and bridge.  
Paul, I'm on Mac too mate and have resolved the issue. See my comment above.

After months of sporadic repros of this issue, I am now certain that VPN use on the music server computer is the culprit. Historically, I hadn't used VPN or extremely infrequently. My usage has gone up considerably in recent months. Basically at the time my Sonos "server cannot be found" issues started. While Sonos often continues to operate for some indeterminate amount of time after the VPN connection is made, at some point it will stop functioning. With moments after killing the VPN connection, Sonos will start functioning again and find my music library.

I've seen on some other VPN threads in this forum that re-opening ports will get Sonos to work again. However, that defeats VPN. That's not an option. I think I have just 2 options: (1) Be aware of the VPN interaction and make do or (2) move the music library to a different computer or NAS.

I have had this same problem consistently so I spent money on a Boost in order to establish a dedicated Sonos network, supposedly more reliable - it didn't help.  I have a Sonos 1, a Connect and a Boost wired into my Apple Time Machine which is, in turn, connected to my ISP's router. My Music Library is sourced from my MacBook Pro.  I have sent a diagnostic report 4434120.  What next?  I'm afraid that I don't fully understand some of the answers above but I have tried changing channels to no effect and I'm fed up with re-loading my music library.

After months of sporadic repros of this issue, I am now certain that VPN use on the music server computer is the culprit. Historically, I hadn't used VPN or extremely infrequently. My usage has gone up considerably in recent months. Basically at the time my Sonos "server cannot be found" issues started. While Sonos often continues to operate for some indeterminate amount of time after the VPN connection is made, at some point it will stop functioning. With moments after killing the VPN connection, Sonos will start functioning again and find my music library.

I've seen on some other VPN threads in this forum that re-opening ports will get Sonos to work again. However, that defeats VPN. That's not an option. I think I have just 2 options: (1) Be aware of the VPN interaction and make do or (2) move the music library to a different computer or NAS.
Hi M. Walter,

VPN was not designed for media streaming by any means, as it brings significant overhead that results in dropouts, name resolution issues and reduced bandwidth.

If a device needs dedicated open ports like for DLNA (which shouldn't be routed through a VPN anyway) the VPN provider has to allow port forwarding through their tunnel, which most VPN services don't allow selectively. 

"Warning" that Sonos may be the cause for the VPN playback issues is more than a stretch.

First thing is make sure the boost is as far away from your router as possible (however long your cable or space allows). 
The Boost is 2.5m from the ISP's router but is sitting on top of the Apple Time Capsule, which is itself linked to the ISP's router (sorry, named it wrongly before). So should I move the Boost away from its current position (and, if so, why?).

The issue is similar to other posts, error message - Unable to play name of track - cannot connect to //name of MacBook Pro because server cannot be found.

The Play 1 is on ground floor and the Boost and everything else is in the same room on the first floor. The Connect (connected to my hi-fi) is 2.5m from the ISP's router and (according to Pythagoras) 3.5m from the Boost and the Time Capsule.