Error on contact form submit after 1.6 update

Hi gravists,

I recently updated a Quark based website from 1.5.x to 1.6 with a modular contact form displaying a message with an Ajax method as explained in the documentation.

The update process broke my submission with a Ooops message and I can’t figure out where’s the issue.

My setup is like this:

A modular “_contact” page with the minimum code:

menu: Contact
cache_enable: false

The form itself in the main modular page:

title: 'Dr DROID'
menu: Home
onpage_menu: true
body_classes: 'title-h1h2 header-dark header-transparent'
    items: '@self.modular'
    name: ajax-form
    action: '/home#contact'
    template: form-messages
    refresh_prevention: true
            name: sujet
            label: Sujet
            placeholder: 'Le sujet de votre message'
            type: select
                1: Information
                2: 'Demande de devis'
                3: 'Demande de rendez-vous'
                4: 'Demander un rappel'
                required: true
            name: name
            label: Nom
            placeholder: 'Entrer votre nom'
            autocomplete: 'on'
            type: text
                required: true
            name: email
            label: Email
            placeholder: 'Saisissez votre adresse email'
            type: text
                rule: email
                required: true
            name: message
            label: Message
            size: long
            placeholder: 'Votre message'
            type: textarea
                required: true
            type: submit
            value: Envoyer
                from: '{{ }}'
                    - '{{ }}'
                    - '{{ }}'
                subject: '[Feedback] {{|e }}'
                body: '{% include ''forms/data.html.twig'' %}'
                fileprefix: feedback-
                dateformat: Ymd-His-u
                extension: txt
                body: '{% include ''forms/data.txt.twig'' %}'
            message: 'Votre message a bien été envoyé. Merci !'

And the mandatory Ajax code in the form.html.twig:

{% set grid_size = theme_var('grid-size') %}
{% set columns = page.header.class == 'small' ? 'col-3 col-md-4 col-sm-6' : 'col-4 col-md-6 col-sm-12'  %}

<section class="section modular-contact {{ page.header.class}} bg-gray">
    <section class="container {{ grid_size }}">
	            {{ page.content }}
        <div class="col-7">
            {% include "forms/form.html.twig" with { form: forms('ajax-form') } %}
		<div class="modal-container">
            <div class="row">
                <div class="col-xs-10 col-xs-offset-1">
                     <div id="form-result"></div>

{% block bottom %}

  var form = $("#ajax-form");
  form.submit(function(e) {

          url: form.attr("action"),
          type: form.attr("method"),
          dataType: "html",
          data: form.serialize(),
          success: function(result) {
          complete: function(result) {
              $("html, body").animate({
                  scrollTop: $("#form-result").offset().top
              }, 200);   
{% endblock %}

This code is working perfectly in my 1.5.x copy of my site but is giving me the “Ooops” error as soon as I update to 1.6 with update of all plugins.

I don’t know how to track the error as I don’t see anything in the log.
How can I debug this ?

EDIT 19/06: I setup a fresh GRAV site, copied the template and the data.
For the moment, with a simple form (without action, template message and AJAX), it’s working. As soon as I try to add functionality, it’s giving me the Oops error.
I’ll investigate a bit more.