Contact form not working properly

Hello,
I’m trying to get the build in contact form running. But it seems to me that the form is working only randomly.
Sometimes it works, sometimes not. I read a lot of discussions about this already. And I’m not the only one with this issue. I’m using enfold on more then 10 pages and I’m wondering how many leads I missed during the last few month…
I’d really like to see an improvement on that.

My issue:
I’m loading a page with the form in an iframe window. Sometimes the form is sent, but there is no message about that appearing. In this case the message is also not sent. Sometimes it works without issues.

I made the following changes in the functions.php:

/*
* Changes the Subject Line of the Enfold Contact Form’s autoresponse.
*/
add_filter('avia_contact_form_args', 'enfold_customization_autoresponsder', 10, 1);
function enfold_customization_autoresponsder( $form_args ) 
{
    $form_args['autoresponder_subject'] = 'Danke für Ihre Nachricht';
    return $form_args;
}

// Uebersetzung und formale Ansprache
add_filter('avf_form_autorespondermessage', 'avf_form_autorespondermessage_mod', 10, 1);
function avf_form_autorespondermessage_mod($message) {
	$message = str_replace('Deine Nachricht:', 'Ihre Nachricht:', $message);
	return $message;
}

// email "von" ändern
function change_cf_from() {
	return "info@DOMAIN.de";
}
add_filter('avf_form_from', 'change_cf_from', 10);

When I add the following code, the autoresponder is not working anymore, even the autoresponder is not sent:

// email "von" ändern
function change_cf_from() {
	return "info@DOMAIN.de";
}
add_filter('avf_form_from', 'change_cf_from', 10);

I need to have the email address of the page, but not the one in the wordpress backend (which is just for administrative issues).

How can I solve this?

Thanks, Enrico