It looks like the Session variable reference you are validating doesn't match the Captcha image setting. Try updating line 17 to:
$WAFV_Errors .= WAValidateLE((strtolower(isset($_POST["Registration_group_5_Security_code_2"])?$_POST["Registration_group_5_Security_code_2"]:"")) . "",((isset($_SESSION["captcha_Registration_group_5_Security_code"]))?strtolower($_SESSION["captcha_Registration_group_5_Security_code"]):"") . "",true,12);