Blazor Server SignalR hub fails on StartAsync due to Azure ADB2C

I also had the same issue and managed to get it working. I had to manually pass through my authentication cookie to the Hub when connecting from my component.

This is my Hub Class:

[Authorize]
    public class NotificationsHub : Microsoft.AspNetCore.SignalR.Hub
    {
        public async override Task OnConnectedAsync() {
            var user = Context.User;
            var identifier = Context.UserIdentifier;
            string name = Context.User.Claims.First(x => x.Type.Equals("name")).Value;
            await base.OnConnectedAsync();
            await Clients.Caller.SendAsync("Connected", 0);
        }
    }

I then configured it in Program.cs

app.UseEndpoints(endpoints => {
    endpoints.MapControllers();
    endpoints.MapBlazorHub();
    endpoints.MapHub<NotificationsHub>("/notificationshub");
    endpoints.MapFallbackToPage("/_Host");
});

I also configured the Hub to use the UserId from my Microsoft cookie:

services.AddSingleton<IUserIdProvider, SignalRUserIdProvider>();

public class SignalRUserIdProvider : IUserIdProvider
    {
        public virtual string GetUserId(HubConnectionContext connection) {
            return connection.User?.FindFirst("http://schemas.microsoft.com/identity/claims/objectidentifier")?.Value!;
        }
    }

Finally, I passed through the cookie when connecting to my Hub. Example component:

@using Microsoft.AspNetCore.SignalR.Client
@using Microsoft.Extensions.Configuration
@implements IAsyncDisposable
@inject IHttpContextAccessor HttpContextAccessor;
@inject NavigationManager Navigate;

@code {
    private HubConnection hubConnection;

    protected override async Task OnInitializedAsync() {
        hubConnection = new HubConnectionBuilder()
            .WithUrl(Navigate.ToAbsoluteUri("/notificationshub"), options => {
                if (HttpContextAccessor.HttpContext != null) {
                    foreach (var cookie in HttpContextAccessor.HttpContext.Request.Cookies) {
                        // IF THE DOMAIN PARAMETER IS WRONG YOU WILL RECEIVE THE JSON ERROR.
                        options.Cookies.Add(new Cookie(cookie.Key, cookie.Value, null, domain: "localhost"));
                    }
                }
            })
            .WithAutomaticReconnect()
            .Build();

        hubConnection.On<int>("Connected", i => {
            // Yay, it worked
            StateHasChanged();
        });

        await hubConnection.StartAsync();
    }

    public async ValueTask DisposeAsync() {
        if (hubConnection is not null) {
            await hubConnection.DisposeAsync();
        }
    }
}

It is a long time since I had this issue, so please do let me know if anything is unclear.

Hope it helps!