Scheduled Backup Always Fails - Error 546

I have scheduled my system to backup weekly using wbadmin START BACKUP
-allCritical -backupTarget:iI -include:C: -quiet

This works fine from a command prompt but it always fails from task
scheduler with result (0xFFFFFFFE). It is running using the administrator
account (for testing purposes) and "run with highest privileges" is checked.

I cannot figure out why this won't work. I also get this error in the
application log:
Log Name: Application
Source: Microsoft-Windows-Backup
Date: 10/31/2007 7:26:50 PM
Event ID: 546
Task Category: None
Level: Error
Keywords:
User: office\Administrator
Computer: office
Description:
The description for Event ID 546 from source Microsoft-Windows-Backup cannot
be found. Either the component that raises this event is not installed on
your local computer or the installation is corrupted. You can install or
repair the component on the local computer.

If the event originated on another computer, the display information had to
be saved with the event.

The following information was included with the event:


The substitution string for insert index (%1) could not be found

Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
<System>
<Provider Name="Microsoft-Windows-Backup"
Guid="{1db28f2e-8f80-4027-8c5a-a11f7f10f62d}" />
<EventID>546</EventID>
<Version>0</Version>
<Level>2</Level>
<Task>0</Task>
<Opcode>0</Opcode>
<Keywords>0x8000000000000000</Keywords>
<TimeCreated SystemTime="2007-10-31T23:26:50.909Z" />
<EventRecordID>4036</EventRecordID>
<Correlation />
<Execution ProcessID="7640" ThreadID="7620" />
<Channel>Application</Channel>
<Computer>office</Computer>
<Security UserID="S-1-5-21-1723699213-1809227743-522139483-500" />
</System>
<ProcessingErrorData>
<ErrorCode>15005</ErrorCode>
<DataItemName>ErrorMessage</DataItemName>
<EventPayload>D0CCAC83151CC8015F0079805F007980</EventPayload>
</ProcessingErrorData>
</Event>