VSTO-specific web request not working (System.Net.WebException)

Question

I wrote a 2013/2016 VSTO app for Microsoft Word using C#. My app creates a new toolbar with buttons. One such button runs my app, which launches a basic Windows Form.

Before the user can work with my app, they need to enter information like their license code and email address. My code in turns sends a basic request to my licensing server and awaits a response.

All my code has been running just fine and now it no longer is. Now, when I run the code, I receive the following two error messages:

System.Net.WebException: 'The underlying connection was closed: An unexpected error occurred on a send.' Inner Exception: IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.

and

System.Net.WebException: 'The underlying connection was closed: An unexpected error occurred on a send.' Inner Exception: SocketException: An existing connection was forcibly closed by the remote host

I decided to run the code using a standard console app to see if I received the same error message, and sure enough, it worked great! Now I am wondering if Word or the Microsoft VSTO technology is blocking my app from accessing my server.

Here is the code in VSTO that does not work Note 1: Created a basic 2013/2016 C# VSTO add-in, added a toolbar, and added Note 2: Added a reference to System.Web. Note 3: Modified the website link and the query strings as I did not want to publish them on this public forum.

using System;
using Microsoft.Office.Tools.Ribbon;
using System.Web;
using System.Net;

namespace WordAddIn3
{
    public partial class Ribbon1
    {
        private void Ribbon1_Load(object sender, RibbonUIEventArgs e)
        {

        }

        private void button1_Click(object sender, RibbonControlEventArgs e)
        {
            // Attempt to activate the product using the licensing server on the website.
            Console.WriteLine("** ActivateLicense");

            //build the url to call the website's software licensing component.
            var builder = new UriBuilder("https://validwebsite.com");
            builder.Port = -1;
            //build the query string.
            var query = HttpUtility.ParseQueryString(builder.Query);
            query["license_key"] = "validactivationcdode";
            query["product_id"] = "validproductid";
            query["email"] = "validemailaddress";
            builder.Query = query.ToString();
            string url = builder.ToString();
            Console.WriteLine("activation request:");
            Console.WriteLine(url); //display the REST endpoint.

            //make the synchronous call to the web service.
            var syncClient = new WebClient();
            var responseStream = syncClient.DownloadString(url);
            Console.WriteLine("Response stream:");
            Console.WriteLine(responseStream); //display the server json response.
        }
    }
}

Here is what is pretty much the same exact code in a console app that does work Note 1: Created a basic C# console app. Note 2: Added a reference to System.Web. Note 3: Modified the website link and the query strings as I did not want to publish them on this public forum. You will receive an error, but that is due to the sample website not having a licensing server.

using System;
using System.Net;
using System.Web;
namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            // Attempt to activate the product using the licensing server on the website.
            Console.WriteLine("** ActivateLicense");

            //build the url to call the website's software licensing component.
            var builder = new UriBuilder("https://validwebsite.com");
            builder.Port = -1;
            //build the query string.
            var query = HttpUtility.ParseQueryString(builder.Query);
            query["license_key"] = "validactivationcdode";
            query["product_id"] = "validproductid";
            query["email"] = "validemailaddress";
            builder.Query = query.ToString();
            string url = builder.ToString();
            Console.WriteLine("activation request:");
            Console.WriteLine(url); //display the REST endpoint.

            //make the synchronous call to the web service.
            var syncClient = new WebClient();
            var responseStream = syncClient.DownloadString(url);
            Console.WriteLine("Response stream:");
            Console.WriteLine(responseStream); //display the server json response.
            Console.ReadKey();
        }
    }
}

Can you help me determine why the code is no longer working in the add-in where it did before (with no code changes)?

I read a lot online and there seem to be too many reasons why this might happen. As an FYI, the website with the licensing server is running. It is (and always has been) a little slow, but when running the code with VSTO, the response is immediate (suggesting no timeout). The Console code runs and there is never a timeout.. I always get a response from the licensing server.

On another thread for a similar problem, someone recommended running WireShark. I am not really familiar with the product, but during my working console run, I received no error messages and instead I got messages like these:

Standard query 0x626a AAAA mywebsite.com

and

Standard query response 0x626a AAAA mywebsite.com

However, if I run the same code in VSTO, I get additional messages that are errors (this one shows up twice):

TCP 60  443 → 50308 [RST, ACK] Seq=1 Ack=125 Win=32768 Len=0