This is offered to try and help anyone else who has just upgraded their Synology hardware with DSM 7 and transferred users and media folders to the new device via configuration export/import and/or backups.  It may also apply to anyone upgrading DSM 6 to 7.

In my case, I was upgrading hardware and DSM version. Having done the DSM configuration and data imports from the old device, I was unable to add the replacement Music Library on the new NAS to S2 Sonos, Error 900 was reported. 

On the Synology, a log entry showed an SMB1 logon was refused.  This was a complete red herring, as I later discovered.

Windows clients also would not connect to the shared folders.

After a lot of playing around with SMB settings, it transpired from the raw smb connection log on the NAS that the issue was that the connections were refused due to the user not having application permissions.

So if you come across this problem, please check in DSM Control Panel “User & Group” that SMB is allowed in application permission for the user that Sonos S2 signs in with (or for a group which the user is a member of) . DSM 7 incorporates a new SMB app, so the SMB app permissions could not be transferred from DSM 6 during the configuration load.  The default SMB app  permission is (sensibly) to not allow access.

Once done, SMB2/3 authentication proceeded fine on both Sonos S2 and Windows clients.