iTunes Match Music Library Sync Fails - Genius results can't be updated

I subscribed to iTunes Match on Sunday, February 21st. Since then I have not once been able to successfully complete a sync of my (large ~90,000 item) library to the cloud. At some point, often taking many many hours, it always fails with a variation on the same error: "Genius results can't be updated right now. An unknown error occurred". A couple of times the process has managed to get as far as actually uploading tracks, but it usually fails before the matching/uploading steps. After investigation it seems that Genius can't be turned off for a library that has Sync Library turned on.

A further wrinkle is that for the tracks that *have* been matched/uploaded, most of them won't play giving the "This song is not currently available in your country or region" error, although I am hoping this issue might be resolved with a successful full completion of a library sync.

I have an open Support case with Apple, have been on the phone with them a couple of times, and am currently waiting on a response from software engineer team on this, but I thought I'd post a question here to see if anyone has had an issue like this.

Here are the solutions I have tried:

1. Deauthorizing/reauthorizing my computer in the Music app
2. Signing out and back in to my iCloud account in the Music app
3. Restarting all my Apple devices
4. Turning off Sync Library on all Apple devices and then turning it back on for all of them, after restarting
5. Syncing my iPhone and iPad with my Macbook Pro via USB cable
6. Creating a new local admin account on my Macbook, switching my music library to that account and signing into my iCloud account from that local user account's Music app

None of these have fixed the issue.

Any responses are appreciated!

Stu