I have this at the very top of my send.php file:
send.php
ob_start(); @session_start(); //some display stuff $_SESSION[\'id\'] = $id; //$id has a value
When using latest WHM (v66.0.23) you may go to MultiPHP INI Editor choose PHP version and set session.save_path to default i.e. /var/cpanel/php/sessions/ea-php70 instead of previous simple tmp - this helped me to get rid of such errors.