If you want to use this module with 1.5.1 and with Société Générale and some how the return url is "index.php?main_page=atos_response" and not "main_page=checkout_process" you have to change atos_response/header_php.php:
//Remember the data from post
$_SESSION['IMO_ATOS_DATA']=$_POST['DATA'];
/* Transaction approved */
/* Check whether the merchant id is the expected one */
zen_redirect(MODULE_PAYMENT_ATOS_PRODUCTION_MODE == 'false'
? zen_href_link(FILENAME_CHECKOUT_PAYMENT,
'info_message=' . urlencode(MODULE_PAYMENT_ATOS_TEXT_CHECKOUT_DEMO),
'SSL')
: zen_href_link(FILENAME_CHECKOUT_PROCESS, '', 'SSL')); // from checkout success in checkout procees
and modify before_process() from atos.php the line :
$response = $this->decodeResponse($_SESSION['IMO_ATOS_DATA']);
unset($_SESSION['IMO_ATOS_DATA']);