JS（jquery）autosearch没有提取MySQL数据

I want to implement a small user autosearch feature for private messages. I cannot find the reason why the below code is not working. JS Ajax ends up each time in .fail node despite the fact that there is a user in DB that e.g. starts with "m".

js:

    $($userSearchForm).on('keyup', function () {

    var $query = $('#userSearch').val();

    $
        .ajax({
            url: '../../../rest/restEndPoints/Privatemessage.php',
            type: 'POST',
            data: $query
        })
        .done(function (response) {

            console.log(response);

        })
        .fail(function (error) {
            console.log('Username not found', error);
        });

});

Privatemessage.php REST endpoint:

if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST["userSearchForm"])) {

    $query = $_POST["userSearch"];
    $queryResult = Privatemessage::searchByUsername($conn, $query);
    $response = ['success' => $queryResult];
}

class Privatemessage

    static public function searchByUsername(PDO $pdo, $userName) {
        $stmt = $pdo->prepare("SELECT u.username FROM Users u WHERE userName LIKE CONCAT('%',:userName,'%')");
        $result = $stmt->execute([
            'userName' => $userName
        ]);

        $ret = [];

        if ($result === true && $stmt->rowCount() > 0) {
            while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {

                $loadedUsername = new Privatemessage($pdo);
                $loadedUsername->userName = $row['username'];

                $ret[] = $loadedUsername;
            }
            return $ret;
        }
        return null;
    }

HTML on privateMessage.php subpage:

            <form class="form-user-search" id="userSearchForm" action="" method="post">
                <div class="form-group">
                    <label for="userSearch">Find user</label>
                    <input name="userSearch" id="userSearch" class="form-control" placeholder="Find user" autofocus>
                    <input name="newPrvMessage" id="newPrvMessage" class="form-control" placeholder="Type in message" autofocus>
                    <div id="userSearch" style="display:none;"></div>
                    <button class="btn btn-default btn-block" type="submit" id="userSearchButton">Submit new private message</button>
                </div>
            </form>

For some reason, when I look up the request that's being sent, the response is only the below:

Despite "not finding" Class, subpage still pulls correctly messages that have been sent/received (rest.php file includes .php classes / endpoints dynamically, depending on subpage you enter via navbar)