Not enough privileges to start scheduled task?

Hi, my question is pretty simple. What is the minimum permission required to start a scheduled task for creating an image?
My company has bought Acronis True Image server and we would like to periodically create an image of one of our servers. I have created a user in our AD domain (Windows 2003 server) and put him in Domain Users group. Unfortunatelly, this is not enough and the scheduled task will not start (the error is Not Enough Privileges R000101F4). I have added this user account in the Administrators group on the server where True Image is installed, but this is not enough as well.
However, if I use Domain Admin account for starting scheduled task, everything is fine. Of course, this is something I would like to avoid.
Thanks in advance,
Mladen
PS. I have looked in the Acronis True Image Server documentation, but I did not find any information about this.