Entity Framework with B2C Auth redirects to empty page, when called with postman "url-missmatch"

When I try to make the following API call:

https://skillbasedmiddleware.azurewebsites.net/tables/useritem?$filter=(email eq '55@55.de')

I'm redirected to a "logging on" window, but then stays empty.

When I try to make a request to the URL via postman (without any auth token) I am getting:

<input type='hidden' name='error' id='error' value='redirect_uri_mismatch'/>
<input type='hidden' name='error_description' id='error_description' value='AADB2C90006: The redirect URI'https://skillbasedmiddleware.azurewebsites.net/signin-oidc' provided in the request is not registered for the client id 'XXX;. Correlation ID: 586439a6-0be6-45f1-aab2-b382dcb8f935' />

My first question: Is it right that I am redirected and that the window stays blank?
My second question: Why is there a "URI MISSMATCH"?

Here is my API configuration:

{
  "AzureAdB2C": {
    "Instance": "https://xxx.b2clogin.com",
    "ClientId": "yyyy",
    "Domain": "ccc.onmicrosoft.co",
    "SignedOutCallbackPath": "/signout-oidc",
    "SignUpSignInPolicyId": "B2C_1_Client"
  },
  "ConnectionStrings": {
    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=NewDb;Trusted_Connection=True"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*"
}

Here is my API startup code:

var builder = WebApplication.CreateBuilder(args);
var connectionString = builder.Configuration.GetConnectionString("DefaultConnection1");
    
if (connectionString == null)
{
    throw new ApplicationException("DefaultConnection is not set");
}

builder.Services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
                .AddMicrosoftIdentityWebApp(builder.Configuration.GetSection("AzureADB2C"));
    
builder.Services.AddAuthorization(options =>
{
    // By default, all incoming requests will be authorized according to 
    // the default policy
    options.FallbackPolicy = options.DefaultPolicy;
});
    
builder.Services.AddDbContext<MigrationDbContext>(options => options.UseSqlServer(connectionString));
builder.Services.AddDatasyncControllers();
    
var app = builder.Build();
    
// Initialize the database
using (var scope = app.Services.CreateScope())
{
    var context = scope.ServiceProvider.GetRequiredService<MigrationDbContext>();
    //await context.InitializeDatabaseAsync().ConfigureAwait(false);
}
    
// Configure and run the web service.
app.UseAuthentication();
app.UseAuthorization();
app.MapControllers();
app.Run();

Where could the issue be coming from?

If you're trying to make an API then no, it's not right that you're being redirected to B2C, you should be receiving a 401 Unauthorized response instead.

It looks like this is happening because you've configured your API to authenticate as a web app (AddMicrosoftIdentityWebApp), rather than a web API (AddMicrosoftIdentityWebApi).

This link should provide an example of setting that up using B2C but you'll need to:

1. Create an app registration in B2C representing your MAUI app (I think you've already done this)
2. Create an app registration in B2C representing your API
3. Expose permissions from your API app registration that you can use to restrict access in your API code
4. Grant those permissions to your MAUI app registration
5. Configure your API app to authenticate API calls.
6. When the MAUI app requests tokens for your user include your API app registration as an additional scope (you should be told what value to use when exposing permissions in step 3)
7. Pass the access_token you receive for your API with all of your API requests