SignalR .Net Client fails with 500 server error on

I'm trying to implement a chat in my app, with azure asp.net web api on back-end and xamarin ios on front-end.

So on back-end I configure my hub with this lines:

        var hubConfiguration = new HubConfiguration();
        hubConfiguration.EnableDetailedErrors = true;
        app.MapSignalR("/signalr", hubConfiguration);

and here is my hub source:

[HubName("Chat")]
public class Chat : Hub
{

    public Task JoinRoom(string roomName)
    {
        return Groups.Add(Context.ConnectionId, roomName);
    }

    public Task LeaveRoom(string roomName)
    {
        return Groups.Remove(Context.ConnectionId, roomName);
    }

    public Task Send(string message, string room)
    {
        return Clients.OthersInGroup(room).addMessage(message);
    }
}

on xamarin ios client everything is pretty simple too:

[Preserve(AllMembers=true)]
public class Msg
{
    public string txt { get; set; }
}

public class Client
{
    private readonly string _userName;
    private readonly HubConnection _connection;
    private readonly IHubProxy _proxy;

    public event EventHandler<string> OnMessageReceived;

    public Client(string userName)
    {
        _userName = userName;
        _connection = new HubConnection("http://mywebsite.azurewebsites.net/");
        _connection.ConnectionToken = NetManager.Instance.token.access_token;
        _proxy = _connection.CreateHubProxy("Chat");
    }

    public async Task Connect()
    {

        await _connection.Start();

        _proxy.On("messageReceived", (Msg platform, Msg message) =>
            {
                if (OnMessageReceived != null)
                    OnMessageReceived(this, string.Format("{0}: {1}", platform, message));
            });

        Send("Connected");
    }

    public Task Send(string message)
    {
        return _proxy.Invoke("Send", _userName, message);
    }
}

So if I connect to server from ios simulator -- it works fine, but when I try to do this from my ipad device -- it crashes with internal server error on line ('await _connection.Start();')

I have already checked server with debugger, but no exceptions raised there, and logs are clear.

Any ideas how to fix this will be very helpful!

[Upd] Some new server logs:

FROM SIMULATOR:

2015-09-27 03:29:39 IBYB GET /signalr/negotiate clientProtocol=1.4&connectionData=[%7B%22Name%22:%22chat%22%7D]&connectionToken=1qZRVTwNMqgGiI8iPpJ9oaPPCeLhHti3UXZR4HYsw2_7SGzOj44WRt8qzBFPRELZu6zk33-8uS7MNaq5K7N5qA2BR1IgzUf8CP9ihoGbjcwtXpFkdyh5gNqFBTHIRSgc2yto5_AOGUok_opd4B9FjAmOhgQlHF_myf28oBBYJxaXZ5iJOXFpI33k6pmQASRvveW-kBRX_89BF2mxAqFkZmVh3_MCo2gWP-NRZZFtMd8ZoxYHnGhyGNVsiiN1KaTHB1xAakP7HZjLpWg7SigfMvtKW0g3eXBsAr1wCJsAKIRjCaMAQFGV0BkKfYztRXvz4QbSXmIBXpKtviYamOqih4-LQJyywwVNh_Djt9H0wYIZmVO565G4ZNKzQfSkK6jMFQz6GfFf_OSlUJIz-0IXsQ7t2kP5VfqVrRu5KK7pyqtZJE5Y4HikRkh6DP8GIYBiXZclmBrpwWhUYVq5P3J2zhDYDNW2GiB95xnRjzXSjPQ&noCache=ccc35de3-5b7a-49ac-bf89-f15145d2634f&X-ARR-LOG-ID=bce175df-8246-4e75-8887-707a7386e1ee 80 - 89.179.240.94 - - - ibyb.azurewebsites.net 200 0 0 942 1718 1093

FROM DEVICE:

2015-09-27 03:35:02 IBYB GET /signalr/negotiate clientProtocol=1.4&connectionData=[%7B%7D]&connectionToken=YIZWqEe7AHvZHwb_aG7jOA9y_NFwUTBuLWSP46q8yh2rQMcjASbsp7VWlZ0Jzo_Z-n230IlhnOHZKm8kJr72ejLF-4LMopwyfZaWmsKNAy6cTd5uyU-76WoXsd2gpmpEJp8A0vMXe2HeLMIvH2Ckw6NIamEbu_uQvHRplkGeUhqGbTQU04dsU47ksebG_zh9XTtLGY9767CiwCYBg_Zk3aFgfrSvzPBiijfmIP9mUhz2ViAigyPeDeOE6WYRgtkkOIGMXGOoS5vQODHMUtiMaoV-w-jcCWtjHzzaObKNeX6zAsB0aJDc9_7fJAoBER7Jd6g0FOuEDvo8D95f1vA8j2SxbBgR4SFIzBDo_JfzO_TbPA6a2FR-ruw3yZHMidmcz3XQWb3vL5a0BPntzL9MPiVgvuhvkXfiRoDrRbzn2YXSqWrN-eEdjsF_WX-LMUc1JyKkjcHP00EAw3kocDWbnXaPqirsSvC5SZ7KY1u63BU&noCache=daae80e6-d209-42c9-8780-35d00fd8208c&X-ARR-LOG-ID=b1aad3d6-7df8-4828-a89c-665d8b550c0c 80 - 89.179.240.94 - - - ibyb.azurewebsites.net 500 0 0 11369 1676 281

So somehow, don't know why from device sends empty group name (1) :[%7B%22Name%22:%22chat%22%7D] (2) :[%7B%7D]

Seems like json serializer problem, but how can I fix that?

finally got it, it was because of 'link all' option in device build options, so json didn't work well