Secure Storage backups fails with the error message "Secure Storage AD backup failed to copy to Secure Storage. Access Denied" however the backup stored in the Remote storage (DC) location is created correctly.
Currently RMAD doesn't support using a minimum permissions account to copy the backup to a Secure Storage server.
Taking that in consideration, an account with Domain Admin permissions will work correctly.
An Enhancement request has been submitted for RMAD to consider the inclusion of supporting using a minimum permissions for this purpose.
Enhancement ID 402464: Secure Storage to pull backups from DC Storage using minimal account permissions