Manual Backup on UnitiServe gives "The backup destination...is currently offline"

I have a UnitiServe (US) and a Synology DS213AIR NAS drive.  (US NDC 4.0.16.1180, NAS DSM 6.2.1-23824 Update 2)

I'm trying to backup the contents of my US's MQ and LQ directories to the NAS.

Using the US Maintenance -> Backup Monitor -> Manual Backup Settings I select the source drive as E:\ (the only option), Destination Drive as \\DS213AIR\backup and backup type of differential. i click on the start button but after a few seconds the Last Error field states, "The backup destination \\DS213AIR\backup is currently offline".

Yesterday evening I did exactly the same thing apart from specifying the backup type as Full as this was the first time I'd attempted a backup. For some reason(s) instead of backing up the entire MQ and LQ directories it only backed up around half of them, hence my attempt today to perform a differential backup to obtain a full backup of all files.

I've performed a host scan within the Tools -> Music Repositories -> Scannable Network Shares and checked the \\DS213AIR\backup appears in the 'Stores for Ripped Music' (use as backup) which it does.

I can see the contents of \\DS213AIR\backup through W10 file explorer and the device is powered up and connected to my network. Other directories on it are seen as online from the US's Tools -> Music Repositories -> Scannable Network Shares list.

Why does the backup not run ?