Mounting ReadyNAS encrypted backup on Linux

I have an RN214 running 6.10.4.  I have attached an eSATA drive to the NAS, created an encrypted volume on the drive using the ReadyNAS UI, and successfully run a backup to the drive.  I then unmounted the drive, and connected it to an Ubuntu 20.04 system.

The Ubuntu system sees a LUKS header, but is unable to decrypt the drive given the key that the ReadyNAS sent via email:

# cryptsetup isLuks --type luks1 /dev/sdb1 || echo "Not LUKS1"
# cryptsetup --key-file=/root/key-from-email.key open --type luks1 /dev/sdb1 backup
No key available with this passphrase.

I've also tried copying the key file directly from /etc/cryptsetup/encrypt_USB_keys on the device, with no change.

Has anyone been able to make this work?  Thank you!