Error during QM Test Case update REST API call

I've wrote a simple C# program to test QM update using OSLC4NET API and RQMFormSample.cs from the OSLC4NET site; the program reads an existing test case and updates the title, but when the UpdateResources is performed, the server returns the following error:

017-12-10 12:42:36,717 [Default Executor-thread-529 @@ 12:42 mondainiriccardo <No User Agent@127.0.0.1> /qm/oslc_qm/contexts/_ltAEkNnmEeehlbPFkEJIaQ/resources/com.ibm.rqm.planning.VersionedTestCase/_a9lTEtprEee3d-SLPPxrUQ] ERROR com.ibm.rqm.oslc.serv
ce                            - OslcUtils:logMessage AQXCM5011E A server error occurred while running the HTTP operation, PUT, at this URL: https://localhost:9443/qm/oslc_qm/contexts/_ltAEkNnmEeehlbPFkEJIaQ/resources/com.ibm.rqm.planning.VersionedTes
Case/_a9lTEtprEee3d-SLPPxrUQ?oslc.properties=dcterms:title. The response code is 500.
om.ibm.team.repository.common.TeamRepositoryException: false^^http://www.w3.org/2001/XMLSchema#string is not a Boolean

This is my sample code:

using System;
using OSLC4Net.Client.Oslc;
using OSLC4Net.Client.Oslc.Resources;
using System.Net.Http;
using OSLC4Net.Core.Model;
using System.Windows.Forms;

namespace RQMInterface
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        CommonFunctions common = CommonFunctions.Instance;
        private void button1_Click(object sender, EventArgs e)
        {
            String catalogUrl = common.helper.GetCatalogUrl();
            TestCase testcase;
            common.serviceProviderUrl = common.client.LookupServiceProviderUrl(catalogUrl, "Tes Test Interno");
            String urlTestCase = common.serviceProviderUrl.Substring(0, common.serviceProviderUrl.LastIndexOf("/"));
            String queryCapability = common.client.LookupQueryCapability(common.serviceProviderUrl, OSLCConstants.OSLC_QM_V2, OSLCConstants.QM_TEST_CASE_QUERY);
            OslcQueryParameters queryParams = new OslcQueryParameters();
            queryParams.SetWhere("oslc:shortId=\"" + txtCase.Text + "\"");
            OslcQuery query = new OslcQuery(common.client, queryCapability, queryParams);
            OslcQueryResult result = query.Submit();
            HttpResponseMessage response = null;
            String[] lista = result.GetMembersUrls();
            testcase = common.client.GetResource(lista[0], OslcMediaType.APPLICATION_RDF_XML).Content.ReadAsAsync<TestCase>(common.client.GetFormatters()).Result;
            
             testcase.SetTitle(testcase.GetTitle() + " (updated)");
            String updateUrl = testcase.GetAbout() + "?oslc.properties=dcterms:title";
            response = common.client.UpdateResource(updateUrl, testcase, OslcMediaType.APPLICATION_RDF_XML, OslcMediaType.APPLICATION_RDF_XML);
            response.ConsumeContent();
        }
    }
}