Can´t run SignalR with Identity, .Net 7 and Angular 17

Question

If i use a proxy, then SignalR (WebSockets) is working, but not Identity (Cookies). If i don´t use a proxy Identity is working, but not SignalR.

The setup is a little odd in this project and i am not allowed to change it. The login page is served from backend with an MVC view, when logged in, the Angular app is loaded. This have worked fine until i tryed adding SignalR.

var builder = WebApplication.CreateBuilder(args);
...
ConfigureIdentity(builder);
ConfigureMVC(builder);
...

// Angular SPA files will be served from this directory
builder.Services.AddSpaStaticFiles(configuration =>
{
    configuration.RootPath = "ClientApp/dist";
});

ConfigureCORS(builder);

// SignalR
builder.Services.AddSignalR();

var app = builder.Build();
...

app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseSpaStaticFiles(new StaticFileOptions {});

app.UseRouting();
app.UseCors(CORS_ALLOWED_ORIGINS);  

app.UseAuthentication();
app.UseAuthorization();

// SignalR
app.UseWebSockets();
app.MapHub<SignalRMessagesHub>("/signalr-messages", (o) => {
    o.Transports = HttpTransportType.WebSockets; 
});

app.MapControllers();
            
app.UseMvc(); // The login page uses mvc. 

// Serve the angular app and set some options, catch-all route to angular app
// Late in the chain, so that other middleware for serving static files, MVC actions, etc., takes precedence.
app.UseSpa((spa) =>
{
        spa.Options.SourcePath = "../../../Frontend_App";
        spa.Options.DefaultPageStaticFileOptions = new StaticFileOptions
        {
            OnPrepareResponse = (context) =>
            {
                var isAuthenticated = context.Context.User.Identity?.IsAuthenticated ?? false;
                if (!isAuthenticated)
                {
                    context.Context.Response.Redirect("/login");
                }
                
                if (string.Equals(context.File.Name, "index.html"))
                {
                    context.Context.Response.Headers.Add("Cache-Control", "no-cache, no-store, must-revalidate, max-age=0");
                            context.Context.Response.Headers.Add("Expires", "-1");
                }
            }
        };

    if (app.Environment.IsDevelopment())
    {
        spa.UseAngularCliServer(npmScript: "start");
        spa.Options.StartupTimeout = TimeSpan.FromMinutes(10);

        // If i add this line & remove above, then i need to start Angular with "ng serve". 
        // But doing so will make Identity to stop working... and SignalR start working..

        //spa.UseProxyToSpaDevelopmentServer("http://localhost:4200"); 
    }
});

app.Run();

I i have seen working examples with SignalR and Angular but not where they also have Identity and a MVC login page. They all have a diffrent setup where the login page is in the Angular app and also uses JWT.

I don´t understand why SignalR can´t connect with frontend. Anyone that can point me in the right direction to solve this?