MQTTNet with MQTT.js are not working together - asp.net-core

Question

I'm trying to use MQTTNet as service broker, that takes requests from webClients over MQTT.js. However, this method is notworking for unknown reasons.
when i test the Service Broker using the windows application "MQTT Explorer" as a client, it works fine.
When i test the MQTT.js Client to connect to an open Service Broker like broker.emqx.io it works also fine.
but the connection between my service Broker with the mqtt client has always a problem. The following error is thrown from the MQTTNet Server:
Client '[::1]:58434' accepted by TCP listener '[::]:8883, ipv6'.
Expected at least 21538 bytes but there are only 69 bytes
MQTTnet.Exceptions.MqttProtocolViolationException: Expected at least 21538 bytes but there are
only 69 bytes
at MQTTnet.Formatter.MqttBufferReader.ReadString()
at MQTTnet.Formatter.MqttPacketFormatterAdapter.ParseProtocolVersion(ReceivedMqttPacket
receivedMqttPacket)
at MQTTnet.Formatter.MqttPacketFormatterAdapter.DetectProtocolVersion(ReceivedMqttPacket
receivedMqttPacket)
at MQTTnet.Adapter.MqttChannelAdapter.ReceivePacketAsync(CancellationToken cancellationToken)
at MQTTnet.Server.MqttClientSessionsManager.ReceiveConnectPacket(IMqttChannelAdapter
channelAdapter, CancellationToken cancellationToken)
at MQTTnet.Server.MqttClientSessionsManager.HandleClientConnectionAsync(IMqttChannelAdapter
channelAdapter, CancellationToken cancellationToken)
Client '[::1]:58434' disconnected at TCP listener '[::]:8883, ipv6'.
configuration of my server are as following:
static async Task<MqttServer> StartMqttServer(bool isDevelopment, ConsoleLogger consoleLogger = null)
{
MqttFactory mqttFactory = new MqttFactory();
if (consoleLogger != null)
{
mqttFactory = new MqttFactory(consoleLogger);
}
// Due to security reasons the "default" endpoint (which is unencrypted) is not enabled by default!
var mqttServerOptions = mqttFactory.CreateServerOptionsBuilder()
.WithDefaultEndpoint()
.Build();
var server = mqttFactory.CreateMqttServer(mqttServerOptions);
await server.StartAsync();
return server;
}
does anybody know why is this happening? And perhaps have an idea how i can fix it?
Thanks in advaced.