Error “EFS-encrypted files may be unusable once restored”
You may see a warning of this form in the backup job logs:
EFS-encrypted files may be unusable once restored, unless you also backup the EFS encryption keys from this PC. To disable this warning, please ensure you have backed up the EFS encryption keys, and then enable the 'I confirm EFS keys are exported' option in the Protected Item settings.
EFS is a Windows feature that allows you to encrypt individual files on disk. The backup job was successful, but if you restore the data to a new PC, the files might not be readable because the EFS encryption keys are tied to the Windows user account. In effect, the backup might not be restorable in a practical sense.
For more information, please see the full article on EFS in the documentation section,