Signalr connection failed for API hosted on webgarden

Question

API serving signalr service (websockets only) is hosted on IIS (webgarden - 3 worker processes, one server) and is configured with the redis backplane:

services.AddSignalR(options =>
            {
                options.EnableDetailedErrors = true;

            })
            .AddStackExchangeRedis(async options =>
            {
                options.ConnectionFactory = async writer =>
                {
                    var config = new ConfigurationOptions()
                    {
                        AbortOnConnectFail = false
                    };
                    config.EndPoints.Add("address", port);
                    config.Ssl = false;
                    config.SyncTimeout = 2000;
                    config.DefaultDatabase = x;
                    config.ClientName = "api1";
                    config.ChannelPrefix = "api1";

                    var connection = await ConnectionMultiplexer.ConnectAsync(config, writer);
                    connection.ConnectionFailed += (_, e) =>
                    {
                        Log.Information ("Connection to Redis failed.");
                    };

                    connection.InternalError += (_, e) =>
                    {
                        Log.Information("Connection to Redis failed with internal error.");
                    };

                    if (!connection.IsConnected)
                    {
                        Log.Information("Did not connect to Redis.");
                    }

                    if (connection.IsConnected)
                    {
                        Log.Information("Connected to Redis.");
                    }
                    return connection;
                };
            })

Web application communicates with that signalr service (via angular). App uses the following connection properites:

skipNegotiation: true,
transport: WebSockets

After authorization there is a connection with valid connectionId. Problem occurs (sometimes) after refreshing that page and I receive the following error message:

WebSocket failed to connect. The connection could not be found on the server, either the endpoint may not be a SignalR endpoint, the connection ID is not present on the server, or there is a proxy blocking WebSockets. If you have multiple servers check that sticky sessions are enabled.

Did I miss something in my configuration? In the log file I see only "Connected to Redis." according to the code I have posted.