Blazor Server Role based authorization in Windows AD - asp.net-core

Question

Since days I try to get authorization working for my Blazor site based on AD group memebership but I cannot make it work.
I am using .NET Core 6 on IIS (IIS Express for testing) in an Windows Domain. I am logged in as AD user and authentication works, so my Blazor page is able to identfy myself.
Now I want to restrict access to some controllers or APIs to specified AD groups and
finally I found a policy option
builder.Services.AddAuthorization(options =>
{
options.AddPolicy ( "MyRole", policy => policy.RequireRole(#"Domain\Group"));
});
To check if the authorization is working I tried
<AuthorizeView Policy="MyRole">
<p>You are in MyRole</p>
</AuthorizeView>
and I also tried
[Authorize(Policy = "MyRole")]
public class MyWebAPIController: ControllerBase
{
...
}
Both tells me that I am not authorized altough I am a member of the specified AD group
This is my startup script:
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddAuthentication(NegotiateDefaults.AuthenticationScheme).AddNegotiate();
builder.Services.AddAuthorization(options =>
{
options.AddPolicy ( "MyRole", policy => policy.RequireRole(#"Domain\Group"));
});
builder.Services.AddRazorPages();
builder.Services.AddServerSideBlazor();
var app = builder.Build();
if (!app.Environment.IsDevelopment())
{
app.UseExceptionHandler("/Error");
app.UseHsts();
}
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthentication();
app.UseAuthorization();
app.MapBlazorHub();
app.MapFallbackToPage("/_Host");
app.UseEndpoints(endpoints =>
{
endpoints.MapControllers(); //<======= this is the line you need to add
endpoints.MapBlazorHub();
});
app.Run();
Can anyone tell me if there is something missing or what I am doing wrong?
Thanks !!