Elgg 1.8.19 - OAuth Plugin 0.10.5 - request token

Question

I am trying to write an Android app to get information from Elgg by using OAuth. I am following this tutorial, whose code is available here.

I am facing a problem when retrieving the request token. I am using the signpost library. The following statement in my app throws an exception :

  // consumer is of type : oauth.signpost.OAuthConsumer
  // provider is of type : oauth.signpost.OAuthProvider

  String url = provider.retrieveRequestToken(consumer, "elgg://oauth");

The exception is :

  Request token or token secret not set in server reply. 
  The service provider you use is probably buggy.

consumer was constructed as :

  consumer = new CommonsHttpOAuthConsumer("<some_key>", "<some_secret>");

provider was constructed as :

 String BASE_URL = "http://<ip-address>:80/elgg" ;
 String REQUEST_URL = BASE_URL + "/pg/oauth/requesttoken";
 String ACCESS_URL = BASE_URL + "/pg/oauth/accesstoken";
 String AUTHORIZE_URL = BASE_URL + "/pg/oauth/authorize";

 provider = new CommonsHttpOAuthProvider(REQUEST_URL, ACCESS_URL, AUTHORIZE_URL);

I found this question posted here, but no answers.

Does anyone know how to resolve this problem ?

I am wondering if it has to do with the following sentence from this page :

 Using your Consumer -

 You use the consumer just as you would any other OAuth consumer. 
 The plugin works by hooking into the PAM system within Elgg and 
 validating OAuth credentials, if they exist on the query.

Do I need to add any additional PAM plugin for Elgg, like this one (it's version seems outdated) ?

Edit:

I do have the URL Getter plugin installed in Elgg, upon which the OAuth plugin is dependent.