VMware vSphere VMs hosted on Cisco HyperFlex access the storage with the help of special Cisco HyperFlex HX Data Platform controllers. The controllers are dedicated VMs deployed on the same VMware ESX host that hosts the VMs.
By default, the controllers are configured to receive traffic only from the ESX(i) host where they are located. To provide communication between Veeam Backup & Replication and the Cisco HyperFlex storage system, you need to manually enable the Direct NFS access mode between the backup proxy and controllers.
For more information about configuring Direct NFS Access on Cisco HyperFlex earlier than 2.0, see this Veeam KB article.