Cannot Run Package In Job Agent With Script Task Included

Dear,

Since I met the strange thing these days. I cannot make it clear that how can I run the SSIS Package in job agent? I hv tried the solutions within this forum, but still couldn't solve my problem.

I have set the right credentials with the admin role of SQL account. In my SSIS package, there is a script task which is used to find out the target flat file as connection string by a user defined variable.

Aspect 1:
If I disable this script task in my SSIS package, then choose "SQL Server Integration Services Package" as the step in Job Agent, run as a proxy, it can run this package without error.

But
if I enable the script task, then choose "SQL Server Integration
Services Package" as the step in Job Agent, run as a proxy. then the
error occurred:

Message
Executed as user: xxxxxxRpt_User. The package execution failed. The step failed.

Aspect 2:Enable the Script task, try to run the package with Dtsexec.exe in Dos command prompt window, it also works:
DTEXEC.exe /FILE "D:ProjectsBBIBBIDMBBI_SSISBBI_ACQ.dtsx" /MAXCONCURRENT " -1 " /CHECKPOINTING OFF
If use the "Operating System (CmdExec)" as the step in Job and type the same command, it will output the error by Job Agent after running:

Executed as user: xxxxxRpt_User. ....00.1399.06 for 32-bit Copyright (C) Microsoft Corp 1984-2005. All rights reserved. Started: 10:28:07 AM Progress: 2006-09-13 10:28:08.10 Source: Import Daily Data Validating: 0% complete End Progress Progress: 2006-09-13 10:28:08.18 Source: Import Daily Data Validating: 50% complete End Progress Progress: 2006-09-13 10:28:08.18 Source: Import Daily Data Validating: 100% complete End Progress Progress: 2006-09-13 10:28:08.20 Source: Transform Data to BBI_ACQ Validating: 0% complete End Progress Progress: 2006-09-13 10:28:08.21 Source: Transform Data to BBI_ACQ Validating: 33% complete End Progress Progress: 2006-09-13 10:28:08.21 Source: Transform Data to BBI_ACQ Validating: 66% complete End Progress Progress: 2006-09-13 10:28:08.21 Source: Transform Data to BBI_ACQ Validating: 100% complete End Progress Error: 2006-09-13 10:28:08.25 Code... Process Exit Code 1. The step failed.

How come? The questions I want to ask Microsoft:
1. Why not keep the simple rule to run SSIS as the former DTS? Why need so much setting before running the package?!!!
2. Why does it prompt the error if a Script Task is included in a SSIS Package? But it works in Dos Command window with dtsexec.exe?

I would like to hear from all buddies here for the solutions. Many thanks for your  help.

Rickey