Launching Exchange Mass Export via the Scheduler

Hello,

I have installed this very useful script (MassExport.ps1) on an Exchange server. 
Related links : MassExport on www.stevieg.org  &   MassExport on Technet

Launched from a Command Prompt (using administrator account), the bat script it works fine.
I also executed the command "New-ManagementRoleAssignment –Role "Mailbox Import Export"–User <Your Admin User>"

However, I cannot run it from the Scheduler; the script always stop after a few seconds...

I tried many configurations :
- Launch it using Administrator account (even not connected) or System account
- Use of "Run with highest privileges"
- Launch a bat script, or only Powershell with full command in Argument
- With or without "Start in" parameter

And nothing works...

Could you please help me to have this script scheduled ?

Thanks in advance.