406 Error when receiving String

I am using this code to send a request from Android to a Spring server. The method in Spring must return a String, but doesnt works:

idstringtoparse=CustomHttpClient.executeHttpPost(urlGetUserIdByUsername, params);

Being the method executeHttpPost this way:

public static String executeHttpPost(String url, ArrayList<NameValuePair> postParameters) throws Exception {

     BufferedReader in = null;
     try{
         HttpClient client = getHttpClient();
         HttpPost request = new HttpPost(url);           
         UrlEncodedFormEntity formEntity = new UrlEncodedFormEntity(postParameters,"UTF-8");                         
         formEntity.setContentEncoding(HTTP.UTF_8);
         request.setEntity(formEntity);
         request.setHeader("Content-Type",
                    "application/x-www-form-urlencoded;charset=UTF-8");     
         HttpResponse response = client.execute(request);
         in = new BufferedReader(new InputStreamReader(response.getEntity().getContent()));      
         StringBuffer sb = new StringBuffer("");
         String line = "";
         String NL = System.getProperty("line.separator");
         while ((line = in.readLine()) != null) {
              sb.append(line + NL);
         }

         String result = sb.toString();

         return result;
     }
}

In the server, the code is like this:

@RequestMapping ("usuario/getIdUserByUsername")
@ResponseBody
public Long getUserIdByUsername(@RequestParam String username){

    Long id=(long)usuarioService.getUserIdByUsername(username);
    usuarioService.getUserIdByUsername(username);       
    return id;
}

This way, I am receiving a 406 error saying "The resource identified by this request is only capable of generating responses with characteristics not acceptable according to the request "accept" headers"

Its a bit strange, since I am using that very same method in my app before this point to log ing the server, and it is working perfectly.