PHPMailer, jQuery + AJAX — обработка формы

Всем привет! Делаю обработку формы с помощью AJAX, при отправке формы я получаю следующую ошибку из JS дебага:   {"readyState":4,"responseText":"","status":200,"statusText":"parsererror"}

Спустя часы сёрфинга гугла и stackoverflow, я так и не нашёл решения своей проблемы, но пришёл к выводу, что ошибка на стороне PHP обработчика. Буду благодарен за любую помощь связанную с данной темой, потому что уже всю голову сломал и не очень понимаю в чём проблема ;(

К этому выводу я пришёл потому что если в методе success в AJAX-запросе указывать data, и если смотреть вкладку Network там всё нормально вроде как..

PHP-код:

<?php

use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;

require_once('PHPMailer/Exception.php');
require_once('PHPMailer/PHPMailer.php');
require_once('PHPMailer/SMTP.php');

if(empty($_SERVER['HTTP_X_REQUESTED_WITH']) || $_SERVER['HTTP_X_REQUESTED_WITH'] != 'XMLHttpRequest') {
    exit();
}
if ($_SERVER['REQUESTED_METHOD'] != 'POST') {
    exit();
}

if($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST['name']) && isset($_POST['message'])) { 

    function secure_input($data)
    {
        $data = trim($data);
        $data = filter_var($data, FILTER_SANITIZE_STRING);
        $data = str_replace(array("\r", "\n"), array(" ", " "), $data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
    }

    $name = secure_input($_POST["name"]);
    $message = secure_input($_POST["message"]);

    if(!empty($_FILES['attachment']))
    {
        $file_name = $_FILES['attachment']['name'];
        $file_tmp_name = $_FILES['attachment']['tmp_name'];
        $file_size = $_FILES['attachment']['size'];
        if($file_size < '2048000')
        {
            $file_path = move_uploaded_file($file_tmp_name, $file_name);
        }else{
            print json_encode(['error'=>1,'msg'=>"Oops! File upload size exceeded...", $file_size]);
            exit;
        }
    }

    $mail = new PHPMailer(true);
    $mail->CharSet = 'UTF-8';
    $mail->IsSMTP();
    $mail->SMTPDebug = 4;
    $mail->SMTPAuth = TRUE;
    $mail->SMTPSecure = "ssl";
    $mail->Port     = 465;
    $mail->Username = "smtp.user";
    $mail->Password = "smt.pass";
    $mail->Host     = "smtp.domain.com";
    $mail->Mailer   = "smtp";
    $mail->SetFrom("email@domain.com");
    $mail->AddReplyTo($_POST["email"]);
    $mail->AddAddress("my@gmail.com");
    $mail->IsHTML(true);
    $mail->Subject = ("MyForm Subject");
    $mail->Body = "
    <h1>Hello dear admin!</h1>
    <p>I am your personal mail delivery robot.</p>
    <b>I have something new for you submitted from the form myForm</b>
    Message: <p>$message</p>
    ";
    $mail->WordWrap   = 80;
    $mail->MsgHTML($_POST["message"]);

    if (is_array($_FILES['attachment'])) {
        $mail->AddAttachment($file_tmp_name, $file_name);
    }
        if (!$mail->send()) {
            $message = 'Error';
        }else{
            $message = 'Email sent successfully';
        }
    }
    $response = ['message' => $message];
    header('Content-type: application/json');
    echo json_encode($response);

jQuery-код:

$(document).ready(function() {
            $("#submitMyForm").click(function(e) {
                e.preventDefault();
                jQuery.validator.addMethod("validDate", function(value, element) {
                    return this.optional(element) || moment(value, "MM/DD/YY").isValid();
                }, "Please enter a valid date in the format MM/DD/YY");
                jQuery.validator.addMethod("phoneTest", function(value, element) {
                    if (/\(?([0-9]{3})\)?([ .-]?)([0-9]{3})\2([0-9]{4})/.test(value)) {
                        return true;
                    } else {
                        return false;
                    };
                }, "Invalid phone number");
                $("#myForm").validate({
                    rules: {
                        scheduleAudience: {
                            required: true,
                        },
                        date: {
                            required: true,
                            validDate: true
                        },
                        time: {
                            required: true
                        },
                        fName: {
                            required: true,
                            lettersonly: true,
                            minlength: 2,
                            maxlength: 30
                        },
                        lName: {
                            required: true,
                            lettersonly: true,
                            minlength: 2,
                            maxlength: 30
                        },
                        email: {
                            required: true,
                            email: true
                        },
                        phone: {
                            required: true,
                            phoneTest: true
                        },
                        preferences: {
                            required: true
                        },
                        message: {
                            required: true,
                            minlength: 20,
                            maxlength: 500
                        },
                        messages: {
                            fName: {
                                minlength: "Please, provide information with minimum - 2 Characters",
                                maxlength: "Please, provide information with maximum - 30 Characters"
                            },
                            lName: {
                                minlength: "Please, provide information with minimum - 2 Characters",
                                maxlength: "Please, provide information with maximum - 30 Characters"
                            },
                            message: {
                                minlength: "Please, provide information with minimum - 20 Characters",
                                maxlength: "Please, provide information with maximum - 500 Characters"
                            }
                        },
                    }
                });

                if ((!$('#myForm').valid())) {
                    return false;
                }
                if (($('#myForm').valid())) {
                    var formData = $("#myForm").serialize();
                    var URL = $("#myForm").attr("action");
                    $.ajax({
                        url: URL,
                        type: "POST",
                        data: formData,
                        cache: false,
                        processData: false,
                        contentType: false,
                        success: function(data) {
                            alert(JSON.stringify(response));
                            if (response.status == 1) {
                                swal({
                                    title: "Good job!",
                                    text: "We will get in touch with you soon",
                                    icon: "success",
                                });
                            }
                        },
                        error: function(response) {
                            alert(JSON.stringify(response));
                        }
                    });
                }
            })
            $("#attachment").change(function() {
                var file = this.files[0];
                var fileType = file.type;
                var match = ['image/jpeg', 'image/jpg', 'image/png'];
                if (!((fileType == match[0]) || (fileType == match[1]) || (fileType == match[2]))) {
                    swal({
                        title: "Invalid file format",
                        text: "Sorry, only JPEG, JPG and PNG files are allowed to upload!",
                        icon: "error",
                    });
                    $("#attachment").val('');
                    return false;
                }
            });
        });

HTML-форма (инпуты name совпадают 100%, во вкладке Network информация в Payload/FormData есть).

<form class="contact100-form validate-form" id="myForm" name="myForm" action="URL_TO_PHPSCRIPT" method="post">

SMTP-данные 100% корректные, если я использую например форму отсюда https://phppot.com/jquery/jquery-contact-form-with-attachment-using-php/  письма отправляются, а в моём коде видимо какая-то проблема.

Валидация на стороне клиента работает без проблем.

Благодарю всех за внимание и буду очень благодарен за любую помощь и подсказки.

P.S. , я сократил количество переменных в PHP коде, чтобы было удобнее читать код, с переменными и их name точно проблем нет, перепроверял и не раз.