Exchange 2010 backups failing with "Machine 'XXXXX' has gone down" error message. (4045316)

The following errors occured after the Exchange database has been mounted and found the required VSS Writers:

1] Background 2011/10/01 12:12:35 0 System VC Machine 'EXCAASK' has gone down
2] Error 2011/10/01 12:12:35 39 Jobs VC Child Process died unexpectedly
3] Error 2011/10/01 12:12:35 39 Jobs VC Job Status: Backup Failed

The Correct VSS Writers are listed in the writers.txt file and were stable.
Further Trace Diagnostics...

From the Job Manager Trace on Server:
0 NET ??? 4888 84 0 94904532875 Connection down
2 JOBMGR ??? 4888 62 0 94943517250 Plugin is now in state Creating the VSS Snapshot
2 JOBMGR ??? 4888 341 0 95023017250 Got PMGR_PROCESS_DIED_MSG
2 JOBMGR ??? 4888 161 0 95023017250 Plugin exited (not expected)
2 JOBMGR ??? 4888 219 0 95023017250 Set exit status to 2
2 JOBMGR ??? 4888 168 1 95023017250 Fatal error: Backup Died Unexpectedly
2 JOBMGR ??? 4888 96 0 95023017250 Starting shutdown
2 JOBMGR ??? 4888 97 0 95023017250 No active plugins, aborting immediately
2 JOBMGR ??? 4888 292 0 95023017250 Send PMGR_KILL_YOURSELF_MSG to all 'children'
2 JOBMGR ??? 4888 99 0 95023017250 Job has finished
2 JOBMGR ??? 4888 100 0 95023017250 Report job status 'Backup Failed' to schedule manager
2 JOBMGR ??? 4888 85 0 95023017250 Waiting for ack from schedule manager
0 MSGWAIT ??? 4888 16 0 95023017250 Message 'MSG_CHANNEL_DOWN_MSG' not handled a depth 0
2 JOBMGR ??? 4888 370 0 95023032875 Message loop completed
2 JOBMGR ??? 4888 185 0 95023032875 Terminal plugin status: 'Creating the VSS Snapshot'

From the Network Manager Trace on Server:
0 NETLOOP ??? 688 53 0 95053064125 Connection to EXCAASK has timed out in state CONNECTING
0 NETLOOP ??? 688 132 0 95053064125 Inform communication manager that connection to EXCAASK is down

From the NetVault Exchange Plugin Trace on Client, after

From the above errors we can see that the communication is lost from the Exchange Client and not been able to communicate from the Client to the Server and / or Server not receiving the handshake that the VSS Snapshot has been created and can carrying on with the backup process.
NetVault Server communicates to the Client via port 20031 and 20032 on UDP and TCP, as well as vice-versa. Then the Clients communicate via a random port to the backup device in order to backup the data.
If any of these channels are disconnected or not received in time, then the backup will fail.

Customer confirmed that had Eset Smart Security 4 AntiVirus Software installed and running during backups.
When this was disbled, the Exchange backups would run successfully again.

Make sure that any Antivirus is not running during backups.
You could stop the Anti-Virus program before starting the backup and re-enable it after the backup completes. Using a pre-script and post-script in the Advanced Options tab for the backups makes this task much easier.
Further documentation on pre-script and post-scripts are in the NetVault Administrators Guide.
You could also tailor the AntiVirus Software to exclude any NetVault Processes (nv*) and the NetVault directories.

Please consult your AntiVirus Software vendor for further documentation and advice.