Welcome to iMore! Where are you seeing this error? Are you restoring OTA or are you using iTunes? What device are you using?
Have you tried booting into recovery and restoring using iTunes? https://support.apple.com/en-us/HT201263
Was the problem backup taken from a device that had 'Encrypted Backups' turned on in iTunes? That would explain the problem.
I've done every single update and restore on all my iPhones in the past 4 years OTA. I've had no issues at all.