'An unknown error occurred [21]' downloading from Apple Music

Hi,

Hopefully someone can provide some insight into this problem, because it's baffled me.

I have an Apple Music account and download some music to my NAS.

I recently downloaded an album without problem. Today, Music informed me that a Dolby Atmos version of the album was available. I have Dolby compatible equipment, so I followed the instructions to delete the old download and redownload the album.

At which point I kept getting an error message:

There were problems downloading some purchased items.

There were problems downloading some purchased items.

There was a problem downloading “track name / album / artist”. An unknown error occurred (21).

...

Clicking an individual track in the pop-up error just says I should check the network is active (it is) and try again (I have, still doesn't work).

The album plays fine as a streaming piece, it's just downloads that seem to be broken for this album.

So far I have tried the following steps:

• Quitting Music
• Logging out of the Mac
• Restarting the Mac
• Signing out of my account in Music / Signing back in
• Deauthorising / reauthorising the machine
• Deleting the album from my Library and adding it again (it can be added, just not downloaded).

No success.

I've tried downloading other albums from Apple Music - they all download just fine, so I am confident the NAS is working fine and Apple Music can see it okay.

The Mac is up to date with MacOS 15.5.

If anyone has an insight that might help - thank-you!

Mike.