This Knowledgebase article provides information about enabling File and Print Sharing on the Secondary channel NIC.
File and Print Sharing should be enabled on the Secondary channel network card. Without File and Print Sharing enabled, the Primary server cannot authenticate against the Secondary server when trying to map a drive for the backup process. A generic authentication error will result.
If this is seen, please enable File and Print Sharing on the Secondary channel NIC. In addition, enable NetBIOS on the channel NIC (LAN only).