Sir, you are amazing, because you've resolved an issue neither Apple nor Synology could.
The question I posted was an aside for another issue I had since changing then name of my NAS where, every time I connected my iPhone via USB, I kept getting an error reporting my old NAS couldn't be found, which baffled everyone, but when I synced my iPhone, I got this error.
On Apple Support advice, I reinstalled my Mac software once and, more problematically, my iPhone software twice, to no avail. The "NAS couldn't be found" error kept coming back.
Your explanation of this error, once I realised the problem was on the iPhone rather than the Music database, led me to delete the spurious music files (there were 3 of them) from the iPhone resolving this issue, but also resolving the "NAS couldn't be found" issue.
So, in essence, I am eternally grateful. Thank you.