Udało się. Trzeba w:
uverride/classes/Mail.php
zmienić 223 wiersz z:
/* Create mail and attach differents parts */ $message = new Swift_Message('['.Configuration::get('PS_SHOP_NAME', null, null, $id_shop).] '.$subject);
na:
/* Create mail and attach differents parts */
$message = new Swift_Message($subject);