MQTT WSS Server

[WimmerMarkus]

Hello, first of all, I would like to thank you for the first-class framework.
I work on a IOT Project in net7, that needs a Secure Mqtt Server. The connection should work with WSS and TCP.
I have written an Angular APP with the ngx-mqtt (javascsript lib)
The connection over WS and TCP (port 1883) workes, but the Secure WSS not.
I have generate a selfSigned certificate with opensssl.
When this workes i want to generate and import the certificate with letsencrypt if this is possible?

Here is my code

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseKestrel(option =>
                {
                    option.ListenAnyIP(1883, l => l.UseMqtt());
                    option.ListenAnyIP(5001, listenOptions =>
                    {
                        var path = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
                        var certPath = Path.Combine(path, "certificate.pfx");
                        var cert = new X509Certificate2(certPath, "12345");

                        listenOptions.UseHttps(cert);
                    });
                    option.ListenAnyIP(5005); // default HTTP Port
                });
                webBuilder.UseStartup<Startup>();
            });
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, MqttServerController mqttController)
 {
     if (env.IsDevelopment())
     {
         app.UseDeveloperExceptionPage();
     }

     app.UseRouting();

     app.UseEndpoints(
         endpoints =>
         {
             endpoints.MapConnectionHandler<MqttConnectionHandler>(
             "/mqtt",
                 httpConnectionDispatcherOptions => httpConnectionDispatcherOptions.WebSockets.SubProtocolSelector =
                     protocolList => protocolList.FirstOrDefault() ?? string.Empty);
         });

     app.UseMqttServer(
         server =>
         {
             server.ValidatingConnectionAsync += mqttController.ValidateConnection;
             server.ClientConnectedAsync += mqttController.OnClientConnected;
             server.InterceptingPublishAsync += mqttController.InterceptApplicationMessagePublishAsync;
         });
 }

The Error message in Browser is this

What did I forget or done wrong in my implementationm?
Thanks
Markus