I can't immediately tell whether that's a Connect gen 1 or gen 2. Note that Sonos S1 can only support SMBv1. 

But, as you state you’re running S2, and it is starting the processing of the library, and then eventually failing, I’d be looking initially at network disruption issues.

For instance, a simple duplicate IP address issue might manifest in exactly the symptoms your describing, as the Sonos device gets bumped off of the network while communicating with the Synology.

Perhaps a diagnostic, submitted after that failure point, followed by a call to Sonos support would have additional data in it to suggest why the failure has occurred. I’m frequently skeptical of the error codes shown. While they’re often spot on, they’re also occasionally more generic than I’d like. 

Oops, missed that. Thanks Bruce. 

> network disruption issues

Could be. It’s on a small home network where Sonos had no trouble with the QNAP previous device, so it feels more like a device problem to me. I’ve set the Synology device to use static IPs and there are only about ten devices on the network, so I think IP collision is unlikely. But I’m not expert. I will take your advice to reproduce, submit a diagnostic, and contact Sonos support. Thank you.

Just to add, as I had the same issue and opened a call with Sonos support. They mentioned that is must be a permission issue on my network. I removed the linked NAS drive and though hit this issue but could not connect back to Sonos. As I had a VPN locally running on the laptop I switched that off. Now I was able to add the connection, but still when I started the indexing I got this error. I created a second folder and started indexing and hat worked on a smaller set of data. So Sonos recommended to use this new folder instead. I was not convinced that the error is really stating the actual issue as the connection was there and no permission issue at all. 
I have now followed their approach and synced my whole system and it took 3 days. I did it in small chunks of 10-20 folders at a time and then started the indexing for each chunk. On many occasions I got this error and saw that the files were not in Sonos. When I just restarted the indexing over and over it eventually worked and I could see the files in Sonos. So obviously this error message is clearly not stating the actual problem.
Speaking back with Sonos support I mentioned my findings and the eventually agreed. They would need to improve their software and either show a list of files which is about to get indexed and / or show the actual issue as to why it is raising this error. 
To me the developer did not a good job in specifying the error and also to show some additional diagnostics when the error appears. Sonos support said that there is no further details on the submitted diagnostics and they cannot switch on any further “debugging” settings to show more details. 

I’m having the same issue moving NAS. Been a Sonos user over 10 years. Everything updated and running S2 on fixed IP addresses

Started off with a Buffalo linkstation (500GB) with about 100GB of music. Switched to a WD MyCloud (2TB) about 8 years ago, no issues that I can remember when reindexing.

Have just switched to a QNAP (12TB) and now about 150GB of music. I set the Sonos music library to the music folder and it starts indexing then fails telling me the device no longer exists. If I set the music library to a small subset of the music it works. 

Guess i’ll be adding my music bit by bit to help Sonos index it

Why not, the next time an indexing process fails, submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it? I’d think the reason for that failure would show up in the diagnostic logs.

If I were to hazard a guess, the NAS is ‘sleeping’ after some period of time, and stopping the Sonos speaker from finishing indexing properly. But of course I don’t have access to those log files that Sonos would. 

I think that t here is a timeout while processing folders with large numbers of files. One approach would be to break the library into smaller shares, such as ‘music/a_g/...’, ‘music/h_m/...’, etc. Another scheme might be ‘music/pop/...’, ‘music/classical/...’, etc.

Along the way, as the files are processed, there must be a sort or hash process. Depending on the order of the files, processing time can vary greatly. For one very simple sort scheme the worst possible order would be if the files are accidently in reverse order -- there is a large time penalty. If the files are accidentally in order, it is one of the fastest sort schemes. 

It is difficult to control the order of processing because SONOS simply asks the NAS for a list of files and SONOS has no control over the list order. My observation is that SONOS processes the library one share at a time. By limiting the number of files in a share, the risk of a timeout is reduced.