System.Net.Http.HttpClient.PostAsync blocks and never returns

I have a .NET framework Windows Forms application with a form that has this code:

using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace test
{

    public partial class Main : Form
    {

        public int exitCode = 1;
        private Options opts;
        CancellationTokenSource cancellationSource = new CancellationTokenSource();


        public Main(Options opts)
        {
            InitializeComponent();
            this.opts = opts;
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            exitCode = 1;
            cancellationSource.Cancel();
            Close();
        }

        async Task doUpload()
        {
            using (var content = new MultipartFormDataContent())
            {
                List<FileStream> streams = new List<FileStream>();
                try
                {
                    foreach (string fPath in opts.InputFiles)
                    {
                        FileStream stream = new FileStream(fPath, FileMode.Open, FileAccess.Read);
                        streams.Add(stream);
                        content.Add(new StreamContent(stream), fPath);
                    }
                    var progressContent = new ProgressableStreamContent(
                         content,
                         4096,
                         (sent, total) =>
                         {
                             double percent = 100 * sent / total;
                             progressBar.Value = (int)percent;
                         });

                    using (var client = new HttpClient())
                    {
                        using (var response = await client.PostAsync(opts.URL, progressContent, cancellationSource.Token))
                        {
                            if (response.IsSuccessStatusCode)
                            {
                                exitCode = 0;
                            }
                            else
                            {
                                MessageBox.Show(
                                    response.Content.ToString(),
                                    "Error " + response.StatusCode,
                                    MessageBoxButtons.OK, MessageBoxIcon.Error
                               );
                            }
                            Close();
                        }
                    }
                }
                finally
                {
                    foreach (FileStream stream in streams)
                    {
                        stream.Close();
                    }
                }
            }

        }

        private void Main_Load(object sender, EventArgs e)
        {
        }

        private void Main_FormClosing(object sender, FormClosingEventArgs e)
        {
            e.Cancel = !cancellationSource.IsCancellationRequested;
        }

        private void Main_Shown(object sender, EventArgs e)
        {
            doUpload();
        }
    }
}

The ProgressableStreamContent is the same that was given here: C#: HttpClient, File upload progress when uploading multiple file as MultipartFormDataContent

The problem is that the response is never returned. In other words: await for postAsync never completes. Also, the progress callback is never called back. Even if I try to use a POST URL that contains a non-exsitent domain, nothing happens. I guess it is a deadlock, but I don't see how? The async Task's result is never used anywhere and it is not awaited for.

It is different from An async/await example that causes a deadlock because .Result is not used and the method is never awaited for, and also it seems that calling ConfigureAwait(false) ha no effect.

UPDATE: I have created a new github repo for this question, so anyone can test it:

https://github.com/nagylzs/csharp_http_post_example

UPDATE: Finally it works. ConfigureAwait is not needed. All UI update operations must be placed inside Invoke. I have updated the test repo to the working version. Also added TLSv1.2 support (which is disabled by default).