Google captcha v3 error, Invalid key, re-registering key not solves problem

Question

I'm trying to add a ReCaptcha v3 to a site I administer.

When I open a page with a captcha, the captcha block displays "Error: Invalid key".

I've already re-registered the key for this site for the third time, but the error remains. I did everything as in the guide, here are the sources:

                <div class="form-wrp">
                    <script src="https://www.google.com/recaptcha/api.js?render=6LduVFIqAAAAAFOgQFfTVM3UJHBYWNMkbm5Ptohv"></script>
                    <form id="order_mail" action="/order/mail" method="post">
                        <h2 class="center">Обратная связь</h2>
                        <fieldset>
                            <input type="text" name="email" placeholder=" " id="inp4"/>
                            <label for="inp4">Email или телефон для ответа</label>
                        </fieldset>
                        <fieldset>
                            <!--<input type="text" name="message" placeholder=" " id="inp5"/>-->
                            <textarea name="message" placeholder=" " id="inp5" required="required"></textarea>
                            <label for="inp5">Сообщение</label>
                        </fieldset>
                        <fieldset>
                            <div class="g-recaptcha" style="width:304px; margin:0 auto;" data-sitekey="6LduVFIqAAAAAFOgQFfTVM3UJHBYWNMkbm5Ptohv" data-callback="onOrderMailSend" data-action="submit"><!--sekret key --></div>
                        </fieldset>
                        <fieldset>
                            <input type="submit"/>
                        </fieldset>
                    </form>
                </div>

How to fix it?

• I was try to replace open key as random string, then error was not displayed. But captcha solving not possible.
• I was try re-register captcha key. The error remains.