Error to backup VM

Hello, I have a problem with the backup policy for one VM.
This is the error log message:

Type: Error
Date and time: 28/05/2012 15:44:58
Backup plan: Backup-VM2
Task: CED-SERVER - Incremental backup
Code: 11.863.304(0xB50508)
Module: 181
Owner:
Message:
Task 'CED-SERVER - Incremental backup' failed: 'Error occurred while running the backup and recovery engine.
Additional info:
--------------------
Error code: 2
Module: 218
LineInfo: 600f7166ce397df5
Fields:
Message: Error occurred while running the backup and recovery engine.
--------------------
Error code: 23
Module: 83
LineInfo: c61573f663f5d7c4
Fields:
Message: Failed to get access to hard disk '[iscsi_datastore_emc] CED-SERVERE/CED-SERVERE.vmdk'.
--------------------
Error code: 4
Module: 0
LineInfo: c61573f663f5d6ba
Fields:
Message: A file was not found
--------------------'.
Acronis Knowledge Base: http://kb.acronis.com/errorcode/

Event code: 0x00DA0002+0x00530017+0x00000004

and

--------------------------------------------------------------
Log Entry Details
--------------------------------------------------------------
Type: Warning
Date and time: 28/05/2012 15:44:58
Backup plan: Backup-VM2
Task: CED-SERVER - Incremental backup
Code: 5.439.489(0x530001)
Module: 83
Owner:
Message:
VixDiskLibVim: TicketLoadSnapshotCb failure - VixError = 4.
--------------------------------------------------------------

The backups of the others VM are OK.

Someone can help me?

the OS is windows server 2008 R2 and the VM CED-SERVER are in the same LUN of others Machines.