Veeam Backup & Replication fails: Unable to release guest

I am using Veeam Backup & Replication to backup several servers on a small ESX 4.1 cluster.  Until recently all has been well and the backups have succeeded every night, but now it fails when trying to backup one of the Windows 2008 R2 domain controllers.  This is the error:

Error: Unfreeze error: [Backup job failed.
Cannot create a shadow copy of the volumes containing writer’s data.
A VSS critical writer has failed. Writer name: [NTDS]. Class ID: [{b2014c9e-8711-4c5c-a5a9-3cf384484757}]. Instance ID: [{dbd891f5-98f4-400c-88dc-dacc9fbdbc76}]. Writer’s state: [VSS_WS_FAILED_AT_POST_SNAPSHOT]. Error code: [0x800423f4].]

I have also noticed the following appear in the Application Event Log:

Log Name:      Application
Source:        ESENT
Date:          18/09/2012 10:12:10
Event ID:      408
Task Category: Logging/Recovery
Level:         Error
Keywords:      Classic
User:          N/A
Computer:
Description:
lsass (516) Unable to write to logfile \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy54\Windows\NTDS\edb.log. Error -1032 (0xfffffbf8).

I spent a bit of time looking at why the VSS write may have an issue.  The things I tried include:

• disabling a Windows Backup job that runs at a similar time (thought they may have been fighting for VSS)
• running “vssadmin list writers” to see if any of the writers were in an error state

Well, it turns out the the fix was quite simple: restart the COM+ Event System service (which in turn restarts the System Event Notification Service, DHCP Server and DFS Replication).  I was then able to immediately retry the backup job and it completed successfully.