SignedOut show as 404

Answer from Google Bard

The error 404 means that the page "MicrosoftIdentity/Account/SignedOut" cannot be found. This can happen for a few reasons:

The page does not exist.
The page is not accessible to the user.
There is a problem with the routing configuration.

To fix this error, you need to check the following:

Make sure that the page "MicrosoftIdentity/Account/SignedOut" exists.
Make sure that the user has permission to access the page.
Check the routing configuration to make sure that the page is accessible.

If you are still getting the error 404, you can try the following:

Clear the browser cache and cookies.
Try a different browser.
Contact the site administrator for help.

Here are some specific solutions that have worked for other people:

If you are using the Microsoft.AspNetCore.Authentication.AzureAD.UI package, you can fix the error by adding the [AllowAnonymous] attribute to the SignOut action in the AccountController.
If you are using the Microsoft Identity package, you can fix the error by adding the endpoints.MapRazorPages(); method to the Configure method in Startup.cs.

add the missing endpoints.MapRazorPages() in startup.cs works for me

https://nuget.info/packages/Microsoft.AspNetCore.App.Ref/3.1.10

This package was built from the source code at https://github.com/aspnet/AspNetCore/tree/e3187077455f953200e3c930430808a30f48b82e

https://github.com/dotnet/aspnetcore/blob/e3187077455f953200e3c930430808a30f48b82e/src/Mvc/Mvc.RazorPages/src/Builder/RazorPagesEndpointRouteBuilderExtensions.cs#L25

/// <summary>
        /// Adds endpoints for Razor Pages to the <see cref="IEndpointRouteBuilder"/>.
        /// </summary>
        /// <param name="endpoints">The <see cref="IEndpointRouteBuilder"/>.</param>
        /// <returns>An <see cref="PageActionEndpointConventionBuilder"/> for endpoints associated with Razor Pages.</returns>
        public static PageActionEndpointConventionBuilder MapRazorPages(this IEndpointRouteBuilder endpoints)
        {
            if (endpoints == null)
            {
                throw new ArgumentNullException(nameof(endpoints));
            }

            EnsureRazorPagesServices(endpoints);

            return GetOrCreateDataSource(endpoints).DefaultBuilder;
        }

private static void EnsureRazorPagesServices(IEndpointRouteBuilder endpoints)
        {
            var marker = endpoints.ServiceProvider.GetService<PageActionEndpointDataSource>();
            if (marker == null)
            {
                throw new InvalidOperationException(Mvc.Core.Resources.FormatUnableToFindServices(
                    nameof(IServiceCollection),
                    "AddRazorPages",
                    "ConfigureServices(...)"));
            }
        }

 private static PageActionEndpointDataSource GetOrCreateDataSource(IEndpointRouteBuilder endpoints)
        {
            var dataSource = endpoints.DataSources.OfType<PageActionEndpointDataSource>().FirstOrDefault();
            if (dataSource == null)
            {
                dataSource = endpoints.ServiceProvider.GetRequiredService<PageActionEndpointDataSource>();
                endpoints.DataSources.Add(dataSource);
            }

            return dataSource;
        }