Request protocol 'HTTP/1.1' is not supported - Net Framework

[gktval]

I have a grpc server/client connection that works for several hundred people. However, I am running into an issue where there are a few clients that cannot connect and are receiving the error Request protocol 'HTTP/1.1' is not supported. I haven't been able to figure out, yet, why it might work for the majority of people, but a few are having issues.

The client software is in .Net Framework 4.7. Below is the code for creating the channel:

public string Token { get; set; }
_dataServiceAddress = "https://www.mysite.com:5001";

public GrpcChannel CreateDataChannel()
{
    var credentials = CallCredentials.FromInterceptor((context, metadata) =>
    {
        if (!string.IsNullOrEmpty(Token))
        {
            metadata.Add("Authorization", $"Bearer {Token}");
        }
        return Task.CompletedTask;
    });

    var winhttpHandler = new WinHttpHandler()
    {
          ServerCertificateValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator,
    };
    SubdirectoryHandler handler = new SubdirectoryHandler(winhttpHandler, "/MySubDir");

    var httpClient = new HttpClient(handler);

    var channel = GrpcChannel.ForAddress(_dataServiceAddress, new GrpcChannelOptions
    {
        Credentials = ChannelCredentials.Create(new SslCredentials(), credentials),
        HttpClient = httpClient,
    });

    return channel;
}

Any thoughts on what to ask or find out from the user that they would not be able to connect to the gRPC channel?

Their computer is running Windows 10.