Backup Databases Task Failure On Logs Only

I've got a couple of backup database tasks setup to run one after the other. The are set up the same, Alldatabases, backup to a device each, and overwrite every time. The task on databases full works fine but the one on logs fails every time, right away with the following rather esoteric error pulled from logging in the event log.
Event Type: Error
Event Source: SQLISPackage
Event Category: None
Event ID: 12550
Date: 29/01/2008
Time: 09:54:11
User: OILCATSsqlservice
Computer: GUNKVBPROC
Description:
Event Name: OnError
Message: The Execute method on the task returned error code 0x80004003 (Object reference not set to an instance of an object.). The Execute method must succeed, and indicate the result using an "out" parameter.

Operator: OILCATSSqlService
Source Name: Back Up Database Task 1
Source ID: {BF117128-FB4C-470D-AC00-4C68797C2C97}
Execution ID: {450BA60C-2EBF-453B-B3E4-80FBC2042F83}
Start Time: 29/01/2008 09:54:11
End Time: 29/01/2008 09:54:11
Data Code: -1073594108

What is this elusive Execute method that is only relevant to the log aspect of this task?