I am trying to use SaveBinaryDirect to upload a file to a Sharepoint Online library. Everything works perfectly on two different workstations I tried on (Windows 10), but once deployed to our UAT server, SaveBinaryDirect throws an exception with the following error message:

"Unable to connect to the remote server".

For reference, here is the code:

using System;
using System.IO;
using System.Linq;
using System.Net;
using System.Security;
using Microsoft.SharePoint.Client;

namespace testSP
    class Program

    static class Constants
        public const string SharepointURL = "https://***.sharepoint.com/";
        public const string Username = "***@***.com";
        public const string Password = "***";

    static private ICredentials GetSharePointOnlineCredential()
        var loginName = Constants.Username;
        var password = Constants.Password;

        var securePassword = new SecureString();
        password.ToCharArray().ToList().ForEach(c => securePassword.AppendChar(c));

        var credential = new SharePointOnlineCredentials(loginName, securePassword);

        return credential;

    static void Main(string[] args)
            ClientContext context = new ClientContext(Constants.SharepointURL);
            System.Net.ICredentials creds = System.Net.CredentialCache.DefaultCredentials;
            context.Credentials = GetSharePointOnlineCredential();
            context.RequestTimeout = 60000000; // Time in milliseconds

            Console.WriteLine("logged in successfully!");

            string fName = Path.GetFileName("1234.txt");               
            string url = "/***/";

            string fnUrl = url + fName;
            using (FileStream fs = new FileStream("1234.txt", FileMode.Open, FileAccess.Read))
                Microsoft.SharePoint.Client.File.SaveBinaryDirect(context, fnUrl, fs, true);

            Console.WriteLine("uploaded '1234.txt' successfully!");
        catch(Exception ex)
            Console.WriteLine("Failed:" + ex.Message);


Server information:

Windows Server 2012 R2, IIS Version 8.5.9600.16384

There are no firewall rules enforced (exception made for server). We can login and upload a file to Sharepoint successfuly through the browser.

Any help or suggestions would be thoroughly appreciated.

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