SignalR Connection interrupted after 20 minutes

Question

I'm running a .NET Core SignalR Hub as a Windows Service with Kestrel under .NET Core 3.1. It is called by a Javascript SignalR Client with the newest client lib (v. 5.0.9)

Reproducible, after 20minutes the connection is stopped without any reason and reconnected after 2seconds. However, no new data is send on the new connection.

I didn't change any of the default timeout settings.

Errorlog Javascript console

public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddCors();
            services.AddSignalR(hubOptions =>
            {
                hubOptions.EnableDetailedErrors = true;
                hubOptions.MaximumReceiveMessageSize = 1048576; //1MB

            }).AddJsonProtocol(options =>
            {
                options.PayloadSerializerOptions = ServiceTools.GetSerializerOptions();
                options.PayloadSerializerOptions.PropertyNameCaseInsensitive = true;
            });

        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseCors(builder => builder
                .AllowAnyHeader()
                .AllowAnyMethod()
                .SetIsOriginAllowed(_ => true)
                .AllowCredentials()
            );

            app.UseRouting();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapHub<VisuNotificationHub>("/signalr");
            });

          
        }
    }

pfw2.connection = new signalR.HubConnectionBuilder()
            .withUrl(signalrUrl,  ['webSockets', 'serverSentEvents', 'longPolling'])
            .configureLogging(signalR.LogLevel.Debug)
            .withAutomaticReconnect({
                nextRetryDelayInMilliseconds: retryContext => {
                    if (retryContext.elapsedMilliseconds < 10000) {             //<10sec
                        return 1000;                                            //sekündlich
                    }
                    else if (retryContext.elapsedMilliseconds < 60000) {        //<60sec/1min
                        return 10000;                                           //10-sekündlich
                    }
                    else if (retryContext.elapsedMilliseconds < 1800000) {      //<30min
                        return 300000;                                          //5-minütlich
                    }
                    else {                                                      //>= 30min
                        return 900000;                                          //15-minütlich
                    }
                }
            })
            .build();

        pfw2.connection.serverTimeoutInMilliseconds = 60000;

        //Reconnect Handler
        pfw2.connection.onreconnecting(error => {
            
            var reconnectErrorData = [{ Message: `<p style="background:red;color:white;font-size:24px">@ResOperatorDialog.ErrorPleaseRefresh</p>` }];
            console.assert(connection.state === signalR.HubConnectionState.Reconnecting);
            console.log("Error in Hub!", error);
            UpdateTicker(reconnectErrorData);
        });

        pfw2.connection.onreconnected((connectionId) => {
            onHubConnectionDone();
            console.log(connectionId);
        });

        //Closed handler
        pfw2.connection.onclose((e) => {
            var connectioNClosedErrorData = [{Message: `<p style=\"background:red;color:white\">Connection closed". Try refreshing this page to restart the connection.</p>`}];
            UpdateTicker(connectioNClosedErrorData);
            console.log(e);
        });   
       
        //connection Start
        pfw2.connection.start()
        .catch((err) => {
            return console.error(err.toString());
        })
        .then(() => {
            console.log("connected, ID=",pfw2.connection.connectionId);
            onHubConnectionDone();
        });