Internal : Failed to unmount disk volumes of the VM: Internal error initiating unmount operation (4046729)

Date: JULY 2010
Product & Version: 8.5
OS Version: all windows
Application version: Vmware Plugin 2.0

Symptoms:
During a file-level backup, the post backup cleanup process fails. Error as shown in binary log:

Information 2010/09/21 22:37:42 3205 Data Plugin NETVAULT Starting Program 'C:/Program Files (x86)/BakBone Software/NetVault Backup/bin/nvvmwmount.exe'
Warning 2010/09/21 22:37:43 3205 Data Plugin NETVAULT Failed with exit code 1
Warning 2010/09/21 22:37:43 3205 Data Plugin NETVAULT Failed to unmount disk volumes of the VM: Internal error initiating unmount operation
Warning 2010/09/21 22:37:43 3205 Data Plugin NETVAULT Failed to perform post-backup cleanup
Error 2010/09/21 22:37:43 3205 Jobs NETVAULT Fatal error: Backup completed
Warning 2010/09/21 22:37:43 3205 Data Plugin NETVAULT VM snapshot must be manually removed

Sometimes the backup completes with warning , sometimes it fails

Any attempt to manually delete the snapshot using the VI client w