While setting up my music again in the new flat, I found that I couldn’t add my shared music folder using the Sonos desktop controller. I was getting an authentication error saying that access was denied to the folder. I knew I could log in with the same credentials using the PC so it had to be a NAS thing.

For me, it was solved by unticking a box in the NAS administration screens: Network services -> Microsoft Networking -> Advanced options -> Allow only NTLMv2 authentication.

All sorted now.