I'm having a strange error with 'Send' button

I have the following code on my page

    <div id="fb-root"></div>
    <script type="text/javascript">
        (function(d, s, id) {
            var js, fjs = d.getElementsByTagName(s)[0];
            if (d.getElementById(id)) return;
            js = d.createElement(s);
            js.id = id;
            js.src = "//connect.facebook.net/en_US/all.js#xfbml=1&amp;appId=<myAppId>";
            fjs.parentNode.insertBefore(js, fjs);
        }(document, 'script', 'facebook-jssdk'));
    </script>
    <br/>
    <fb:like send="true" width="450" show_faces="true"></fb:like>

The 'send' button works fine if I access the application via IP address, but if I use a domain name, Facebook returns

The page at http://<...>.com:8080/pages/question.jsf could not be reached because the server returned status code 401.

Meantime 'like' button works fine. The application front-end is built with JSF2+Primefaces.

Any ideas would be appreciated Thanks