WebAuthenticationBroker returns a UserCancel WebAuthenticationResult on a successful Facebook authentication - facebook

Question

Using the WebAuthenticationBroker for Single Sign On in a Windows Phone Store app (WinRT / WP8.1), i use the following code to engage the Facebook login process :
string redirectUri2 = Windows.Security.Authentication.Web.WebAuthenticationBroker.GetCurrentApplicationCallbackUri().AbsoluteUri;
FacebookClient fb = new FacebookClient();
Uri loginUri = fb.GetLoginUrl(new {
client_id = FacebookAppID,
redirect_uri = redirectUri2,
response_type = "token",
scope = FacebookScope,
display = "popup"
});
WebAuthenticationBroker.AuthenticateAndContinue(loginUri, new Uri(redirectUri));
In the ContinueWebAuthentication(WebAuthenticationBrokerContinuationEventArgs args) callback, the WebAuthenticationResult.ResponseStatus I get is UserCancel, even on a successful sign in.
Trace of the last response from Facebook :
HTTP/1.1 200 OK
X-Content-Type-Options: nosniff
Cache-Control: private, no-cache, no-store, must-revalidate
Expires: Sat, 01 Jan 2000 00:00:00 GMT
X-XSS-Protection: 0
X-Frame-Options: DENY
Facebook-API-Version: v2.0
Pragma: no-cache
Content-Type: text/html; charset=utf-8
P3P: CP="Facebook does not have a P3P policy. Learn why here: http://fb.me/p3p"
Set-Cookie: wd=deleted; expires=Thu, 01-Jan-1970 00:00:01 GMT; Max-Age=-1413367561; path=/; domain=.facebook.com; httponly
X-FB-Debug: {edited out}
Date: Wed, 15 Oct 2014 10:06:02 GMT
Connection: keep-alive
Content-Length: 411
<script type="text/javascript">window.location.href="ms-app:\/\/s-1-15-2-908865707-3825634006-813379085-3082126904-2549935584-1522363559-xxxxxxxxxx\/#access_token={edited out}&expires_in=5113018";</script>
What is going on with the WebAuthenticationBroker ?
Additional information on the issue : the date/time/timezone on the phone are correct.