unable to play - server cannot be found

First thing is make sure the boost is as far away from your router as possible (however long your cable or space allows). 
Thank you for the explanation Chris.

The Boost is connected wired to the Time Capsule, should I disconnect it and move it elsewhere, I thought that it had to be connected to a router. 

Macbook Pro is operating wirelessly.  It is being used as the music source but I thought that this was only the case to initially load the music library which was then available from Sonos independently.  If not this means Sonos is reliant on the strength/capacity etc. of the home wi-fi network which is, in turn, reliant on the ISP's router strength/capacity etc. etc.

Should I take it then that the following para only applies if the music source is wired to a Sonos component in the "Sonos matrix network"?

DEDICATED SONOS NETWORK BENEFITS
Plugging in a product launches a reliable mesh network on a separate wireless channel, which means that your speakers don’t have to talk to the WiFi router directly, but rather can connect reliably through any nearby Sonos player. This increases the range of your Sonos system, and ensures that you’ve got great connectivity no matter how large your home or how many WiFi devices you use.

It is not practical for me to wire the MacBook Pro to the router every time I want to play music so failing all else it's looking like more expense to get a NAS drive and to wire it into the Sonos network - two questions then: 1. what Sonos component should a NAS drive be connected to? and 2. are all NAS drives much the same?

The Boost you need to connect to your router - just keep it a foot or so away from your router/wifi device.

Sonos does not "load" your music library.  It indexes your library from the location that it is stored.  In your case it is stored on your MacBook.  So when you play a song in Sonos it goes out and has to pull the song from your MacBook.  Your MacBook is connected to your home wifi so it then has to send the song back via your home wifi to your router ... which in turn passes it to the Boost ... which in turn then routes it through Sonosnet.

What you want to do is get an NAS and hook the NAS to your router as well.  Then point sonos to the NAS to index and when you play a song Sonos will pull the song from the NAS drive - directly into Sonosnet (your home wifi is then removed from the equation and you should be good).  Have you noticed you don't have issues if you play an internet streaming service.  Because they filter through your router and direct into Sonosnet as well.

You can get an NAS drive to hook to your router for just over $100 these days http://www.amazon.com/Seagate-Central-Personal-Storage-STCG4000100/dp/B00ARB5FNE/ref=sr_1_3?ie=UTF8&...

Good for music and storing lots of stuff!

Thanks again.

Yes, I had noticed that internet streaming worked fine.

Looks like it's a NAS drive then!

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.
Bernhard, I think you misunderstood and I don't quite appreciate the insinuation that I was attempting to malign Sonos for my finding. I'm not attempting to stream over VPN. My library just happens to be on a PC that I also use periodically to VPN in to work to get my job done. VPN just happens to interfere with my Sonos player from reaching the share on my PC.

If there's anything I would like to see Sonos do is to update controller installation/setup documentation to note that VPN use on the same device hosting the library could interfere with Sonos playback. Some guidance on library configuration might be another (better?) option.

NAS Drive is great to have on your network for saving things to - great for backing up that laptop completely (not just music).

and its a good sign that internet sources work well.  That means your router is not bogged down and SonosNet is working as it should.  Your problem is your home wifi reliability ... which is common and why going with SonosNet as you did (getting boost) is the way to go.

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.
As convenient VPN connection from a client system are they often interfere as the target server uses the same private IP address ranges as your local network at home.

Essentially, your company uses one or several of the IP address ranges below which causes the the traffic being routed to the company server instead of the Sonos. There is little the Sonos app or the Sonos devices can do to alter this.

Long story short, the best you can do is to verify which of the IP ranges your company is using and putting your home network into a different address range.
E.g. if 192.168 ... is used by your company change your router configuration to 10.0 ... to avoid the overlap.

We have some true networking experts here in the community (and so has Sonos) who can assist way better then myself.

Lets see if one of them picks up the thread.

And in the spirit of a good community, my apologies as i misunderstood your configuration.

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 would contact sonos direct, they can probably resolve this for you. http://www.sonos.com/en-gb/contact?r=1

actually I've just realised that I can plug an HDD into the Time Capsule USB port so no need to buy a NAS

I have never used a time capsule but sounds right.  Actually I think the Time Capsule itself can be used - sorry I didn't even think about it.  So you just put music on the time capsule and here I think this thread links to several ways to setup

https://ask.sonos.com/sonos/topics/best_way_to_use_apple_time_capsule_with_sonos_nas