Error when creating Virtual Data Recovery on Server

Hi,

I am getting this error when creating a VDR backup on a Windows 2003 Server. Other VM server works just fine. However, for this server I am getting this error message:

5/24/2010 5:31:09 AM: Normal backup using Backup Job 1

5/24/2010 5:31:12 AM: Failed to create snapshot for 'servername', error -3960 ( cannot quiesce virtual machine)

5/24/2010 5:31:12 AM: Task incomplete

Anybody have any idea what this error ID meant?

Thank you.

Collin