Did you get a Task Scheduler error message;
"An error has occurred while attempting to set task account information.
The specific error is: 0x8007005: Access is denied.
You do not have permission to perform the requested operation."
If so, and you have not set a password for your Admin user account, you need to do so. When you schedule a backup, when you get the "Set Account Information" dialogue box, you need to enter the password in (your user name is automatically filled in for you).
Hope that helps, even if it is some months since you posted your query!!