Error execute Schedule Backup Job

Hi,
I am using MSSQL 2005 and apply SP3. There is no problem found when manual backup
When execute Schedule Backup Job, error message below encounter :
Message :
Executed as user: ..SYSTEM. ...rsion 9.00.4035.00 for 32-bit Copyright (C) Microsoft Corp 1984-2005. All rights reserved.
Started: 11:19:59 AM Error: 2010-01-16 11:20:00.29 Code: 0xC0010018 Source: Back Up Database (Full)
Description: Error loading a task. The contact information for the task is "Back Up Database Task;
Microsoft Corporation; Microsoft SQL Server v9; ? 2004 Microsoft Corporation;
All Rights Reserved;http://www.microsoft.com/sql/support/default.asp;1".
This happens when loading a task fails. End Error Progress: 2010-01-16 11:20:00.76
Source: {44939EC1-CCAE-44E1-87F5-74C905C28529} Executing query "DECLARE @Guid UNIQUEIDENTIFIER
EXECUTE msdb..sp".: 100% complete End Progress Error: 2010-01-16 11:20:01.05
Code: 0xC0024104 Source: Back Up Database (Full)
Description: The Execute method on the task returned error code 0x80004003 (Collection cannot be null.
Parameter name: c). The Execute method... The package execution fa... The step failed.
Any one can help. Thank you.
from : Michelle