Failing to import snapshot: "Error: Manifest data not found in archive"

Hi there - I've been bashing my head against a brick wall trying to get this site migrated and no matter what I do, I get the same error when I try to import the zip archive - "Error: Manifest data not found in archive".

Full output:

Importing archives from: /var/www/

Processing archive: (530.2M)
Error: Manifest data not found in archive.
Snapshot import complete

I've reviewed the many forum posts about this same symptom, but none of those suggestions seem to help. I've opened the archive to check that the manifest file exists, and that it doesn't look malformed. It looks fine to me. Any help will be greatly appreciated.

I've enabled support access to the site so you can have a look.

Thanks in advance,