404 error on subscription

Just as a hint:

Using a customized html form as direct input on the “Subscription page” is currently not a good idea due to this line in subscription.php which makes it impossible to have the form’s action attribute something like http://www.domain.tld/?na=s:

$message = str_ireplace('<form', '<form method="post" action="' . plugins_url('newsletter/do/subscribe.php') . '" onsubmit="return newsletter_check(this)"', $message);

Saving the custom form in the custom forms section and using the short code {subscription_form_1} on the subscription page works great.