Switching to https messed up my ajax calls

Question

since I uploaded my project to a webhosting I am facing the following issue: whenever I send an ajax request I get this error: POST https://_ DOMAIN _ /backend/login.backend.php 500

apparently the google API works as I get all the data on php but the mysql requests never get through. It could be something about switching from http to https as it used to work perfectly on my localhost and now once I uploaded it, it does not work anymore.

            <script src='https://apis.google.com/js/api:client.js'></script>
            <script>
                var googleUser = {};
                var startApp = function() {
                    gapi.load('auth2', function(){
                    auth2 = gapi.auth2.init({
                        client_id: 'MY_ID_IS_HERE.apps.googleusercontent.com',
                        cookiepolicy: 'single_host_origin',
                        //scope: 'https://www.googleapis.com/auth/user.addresses.read',
                    });
                    attachSignin(document.getElementById('customGoogleButton'));
                    });
                };
                function attachSignin(element) {
                    auth2.attachClickHandler(element, {}, onLogin, onFail)
                }

                function onLogin(googleUser) {
                    var profile = googleUser.getBasicProfile();
                    var id_token = googleUser.getAuthResponse().id_token;
                
                    var xhr = new XMLHttpRequest();
                    xhr.open('POST', 'backend/login.backend.php'); // link
                    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
                    xhr.onload = function() {
                        console.log('Signed in:' + xhr.responseText);
                        //window.location.reload();
                    };
                    xhr.send('idtoken=' + id_token);
                }
                function onFail(error) {
                    console.log(error);
                }
                startApp();
            </script>

    require_once $_SERVER['DOCUMENT_ROOT'].'/vendor/autoload.php';
    include_once $_SERVER['DOCUMENT_ROOT']."/db/config.php";

    //$jwt = new \Firebase\JWT\JWT;
    //$jwt::$leeway = 10;

    if (!isset($_POST["idtoken"])) {
        exit();
    }
    $id_token = $_POST["idtoken"];
    $CLIENT_ID = 'MY_ID_IS_HERE';

    $client = new Google_Client(['client_id' => $CLIENT_ID]); 
    $payload = $client->verifyIdToken($id_token);
    if ($payload) {

        //log in
        session_start();
        
        $_SESSION['userFirstName'] = $payload["given_name"];
        $_SESSION['userLastName'] = $payload["family_name"];
        $_SESSION['userFullName'] = $payload["name"];
        $_SESSION['email'] = $payload["email"];
        $_SESSION['user_image'] = $payload["picture"];
        
                
        $email = mysqli_real_escape_string($conn, $payload["email"]);
        $first = mysqli_real_escape_string($conn, $payload["given_name"]);
        $last_name = mysqli_real_escape_string($conn, $payload["family_name"]);
        $user_image = mysqli_real_escape_string($conn, $payload["picture"]);

        $stmt = $conn->prepare('SELECT id from users WHERE email= ?');
        $stmt->bind_param('s', $_SESSION['email']);
        $stmt->execute();
        $result = $stmt->get_result();
        $data = $result->fetch_assoc();
        if (empty($data)) { 
            $sql = "INSERT INTO users (first_name, last_name, user_image, email) VALUES (?, ?, ?, ?);";
        } else {
            $sql = "UPDATE users SET first_name = ?, last_name = ?, user_image = ? WHERE email = ?;";
        }
        mysqli_query($conn, $sql);
        $stmt = mysqli_stmt_init($conn);
        if (!mysqli_stmt_prepare($stmt, $sql)) {
            echo "SQL ERROR";
        } else {
            mysqli_stmt_bind_param($stmt, "ssss", $first, $last_name, $user_image, $email);
            mysqli_stmt_execute($stmt);
        }
        if (empty($data)) {
            $stmt = $conn->prepare('SELECT id from users WHERE email= ?');
            $stmt->bind_param('s', $_SESSION['email']);
            $stmt->execute();
            $result = $stmt->get_result();
            $data = $result->fetch_assoc();
        }
        $_SESSION['user_id'] = $data['id'];
        exit();
    
    } else {
        echo "<h1>ERROR</h1>";
    }