After several years effort I THINK I have cracked network issues that I have been having with Windows and my DS920+ DSM 7.2.1. It never behaved as well as the Server 2012 it replaced. I was getting Error 0x80070035 when PCs tried to access the shares, but either after running the suggested windows diagnostics, or just waiting and trying again it would work. The SYNNAS would be variable about appearing in the network list on Windows clients.

I'm still not sure what the silver bullet setting was, but the prime suspect is the Master Browser, and a Windows 10 bug where the LAN master browser doesn't behave on a Windows 10 PC. It appears the having the SYNNAS as master browser also gives problems.

I have a number of Windows 10 and 11 pcs. On learing about the Windows 10 master browser misbehaving I used the registry setting HKLM\System\CurrentControlSet\Services\Browser\Parameters\MaintainServerList=Off

setting to prevent the Windows 10 and laptops from becoming the master browser. I then stopped the SYNNAS from assuming this role, as that also seemed to be flawed, and the 2 Windows 11 PCs were allowed to negotiate which would do it. That seemed to fix it. It also seems to have fixed SONOS access issues...

Other DS950+ settings that might be germane: 

Maximum SMB SMB3

Minimum SMB SMB2

Transport Encryption - Client defined

Server signing - Disable

Enabled Opportunistic Locking

Enabled SMB2 lease

SMB durable handles - disabled

Master Browser - disabled (in case guest access was part of the issue)

Dirsort - disabled

Symbolic links enabled within shared folders

Monitoring changes on all subfolders - enabled

(all other options on this screen disabled)

On the Advanced tab:

Bonjour service discovery - enabled

SSDP Windows network discovery - enabled

Bypass Traverse checking - enabled

(all other option on this screen disabled)

SMB cache was cleared after all settings changed.

Another suggestion of adding a local group policy setting on all PCs to set LAN Manager authentication level set to Send NTLMv2 response only. Refuse LM & NTLM proved to be a red herring.

I'm adding this to the SONOS forum as SONOS is now working much more reliably, so it looks like the change in settings has relieved some LAN/SYNNAS burdens and improved performance. The SONOS Windows controller now sees all room reliably, and Plex shuffle playing no longer has skipping tracks due to inaccessibility!