[ABR10]: Task 'Full backup' has been canceled on machine

I have a group of PC's that I backup using Acronis Backup & Recovery build 10.0.12703. They store their backup files in unmanaged Centralized Vaults. On the Management Console, I have two different backup policies to create monthly full backups of several PCs. This same problem happens intermittently for both policies. The intermittent problem I'm having is the backup will start and run for awhile and then suddenly quit without any warning on the Management console but email that the [ABR10]: Task 'Full backup' has been canceled on machine. From the Management console if I try to run the backup job again, it may complete successfully or have the same problem of being canceled. I am not canceling the job. Why is this happening?

Here is an example of the email message I receive when the job is cancelled:
Subject: [ABR10]: Task 'Full backup' has been canceled on machine 'CRM1_SVR.pcn.ak'.

1 Information 2/12/2011 5:56:05 PM Priority changed to Low.
2 Information 2/12/2011 5:56:05 PM Analyzing partition '0-0'...
3 Information 2/12/2011 5:56:06 PM Analyzing partition '1-1'...
4 Information 2/12/2011 5:56:06 PM Analyzing partition 'C:'...
5 Information 2/12/2011 5:56:06 PM Analyzing partition 'E:'...
6 Information 2/12/2011 5:56:06 PM Analyzing partition 'C:'...
7 Information 2/12/2011 5:56:06 PM Create Full Backup
From: NTFS (C:)
To file: "file://kocna004\images$\\Ghost%20Images\ALP\GE%20HMIs/GE%20HMI%20Full%20Backup_CRM1_SVR.pcn.ak_Archive%201_2011_02_12_17_56_03_513D.TIB"
Compression: Normal
Exclude: Files matching mask
Match criterion: C:\Documents and Settings\All Users\Application Data\Acronis\BackupAndRecovery\MMS\Temp\acr599.tmp,

8 Information 2/12/2011 5:56:06 PM Pending operation 147 started: 'Creating partition image'.
9 Information 2/12/2011 5:56:07 PM Locking partition C:...
Task 'Full backup' has been canceled on machine 'CRM1_SVR.pcn.ak'.

Here is an example of the email message I receive when the same job completes successfully:
Subject: [ABR10]: Task 'Full backup' has succeeded on machine 'CRM1_SVR.pcn.ak'.

1 Information 2/12/2011 6:03:36 PM Priority changed to Low.
2 Information 2/12/2011 6:03:36 PM Analyzing partition '0-0'...
3 Information 2/12/2011 6:03:36 PM Analyzing partition '1-1'...
4 Information 2/12/2011 6:03:36 PM Analyzing partition 'C:'...
5 Information 2/12/2011 6:03:36 PM Analyzing partition 'E:'...
6 Information 2/12/2011 6:03:36 PM Analyzing partition 'C:'...
7 Information 2/12/2011 6:03:36 PM Create Full Backup
From: NTFS (C:)
To file: "file://kocna004\images$\\Ghost%20Images\ALP\GE%20HMIs/GE%20HMI%20Full%20Backup_CRM1_SVR.pcn.ak_Archive%201_2011_02_12_18_03_35_003D.TIB"
Compression: Normal
Exclude: Files matching mask
Match criterion: C:\Documents and Settings\All Users\Application Data\Acronis\BackupAndRecovery\MMS\Temp\acr59D.tmp,

8 Information 2/12/2011 6:03:36 PM Pending operation 147 started: 'Creating partition image'.
9 Information 2/12/2011 6:03:37 PM Locking partition C:...
10 Information 2/12/2011 6:10:34 PM Pending operation 144 started: 'Saving partition structure'.
Task 'Full backup' has succeeded on machine 'CRM1_SVR.pcn.ak'.

Thanks for your assistance.

Chris