Your hidden form element doesn't have a name... seems like you are trying to use it to pass the file name to be downloaded.
Also your download file name is set to: $_SESSION['members_id']
Seems like that would be set to the posted hidden form name since the file name isn't stored in that session variable.
I'd probably add: name="downloadFile"
to your hidden form element and then update the file name in the download server behavior to: <?php echo($_POST['downloadFile']); ?>