The Scan User Account may not have the Correct Share or Folder Permissions.
Contact the local system administrator to confirm the scan user account has full modify permissions within the scan share directory structure.
Note: Make sure the account being used is listed in both the Sharing and Security tabs as having full control.
Note: When using Win XP you need to turn off Use Simple file Sharing (recommended) in order to see the premissions section of the Sharing tab. This can be found by going to Control Panel and clicking on Folder Options. Then go to the View tab and scroll to the bottom to find the file Sharing Option.