By default, in cross-site XMLHttpRequest invocations, browsers will not send credentials.  A specific flag has to be set on the XMLHttpRequest object when it is invoked.

C'est ce que j'ai fait :

      $.ajax({
        type: 'POST',
        url: 'http://b.epoc.fr/jsonrpc.php',
        data: data,
        username: 'ahah',
        password: '415h6dfh12reh',
        xhrFields: {
          withCredentials: true
        },
        contentType: 'application/json',
        dataType: 'json',
        success: function(response) {
          console.log(response);
        },
        error: function(xhr, type) {

        }
      });
    });

Toujours le même message d'erreur.