[warning] [remote] unable to submit form ; form attached

[Casper Fun]

I have a form as follows:

<form id="login_form" class="initialized" action="https://XXX/login" method="post">

    <div id="login_Container" class="inputcontainer input empty">
        <input id="username" placeholder="username" name="username"></input>
        <span class="message empty" style="height: 0"></span>
        <div class="icon"></div>
    </div>
    <div id="password_Container" class="inputcontainer input empty">
        <input id="password" type="password" placeholder="password" name="password"></input>
        <span class="message empty" style="height: 0"></span>
        <div class="icon"></div>
    </div>
    <button id="sign_in_button" class="flatbutton" type="submit">

        Sign In

    </button>

</form>

In a previous version of the site I was able to submit the form by

  var login = function(){                                                                           
    casper.thenOpen('https://XXX/login');
 
   casper.then(function() {
        this.fill('form#login_form', {
          'username': 'user',
          'password': 'pass'
        }, true);
    });


from the debug log

[info] [remote] attempting to fetch form element from selector: 'form#login_form'
[debug] [remote] Set "username" field value to user
[debug] [remote] Set "password" field value to ****
[info] [remote] submitting form to https://XXX/login, HTTP POST
[warning] [remote] unable to submit form


Am I going about it the wrong way? Can I get more details than [warning] [remote] unable to submit form  ?

[j...@borderxlab.com]

Bump this question. I had a similar issue. Although the form seemed eventually submitted. I doubt what this warning message actually indicates. 

Anyone can help?

Thanks,

Jun