error code 6- Vmbackup failure (virtual backup host)

Hi,

Netbackup Master/Media server version 7.5.0.6
OS : windows server 2008 R2, 64-bit
Backup host : windows 2003 R2, Netbackup 7.5.0.6 , virtual machine.
transport mode NBD

I newly configured snapshot backup for vmware, it is failing continuesly with error code 6,

ERR - Error opening the snapshot disks using given transport mode: Status 23

anyone guide me to fix this issue.

-Thank you