Issues setting up discord oauth in ASP.NET Core MVC

I am (currently) setting up OAuth2 for Discord with https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers and having troubles. It functions well until I click 'authorize' in the authorization window of discord. It should supposedly return to my action with route discord-signin which is also defined in the bots OAuth2 settings. But it does not, instead it redirects without error to my Index-Action.

Here is what I define when adding the discord authentication in Program.cs

builder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
    .AddCookie()
    .AddDiscord(o =>
    {
        o.ClientId = DiscordDefaults.ClientId;
        o.ClientSecret = DiscordDefaults.ClientSecret;
        o.CallbackPath = new PathString("/signin-discord");
        o.AuthorizationEndpoint = DiscordDefaults.AuthorizationEndpoint;
        o.TokenEndpoint = DiscordDefaults.TokenEndpoint;
        o.UserInformationEndpoint = DiscordDefaults.UserInformationEndpoint;
        o.Scope.Add("identify");

        o.ClaimActions.MapJsonKey(ClaimTypes.NameIdentifier, "id", ClaimValueTypes.UInteger64);
        o.ClaimActions.MapJsonKey(ClaimTypes.Name, "username", ClaimValueTypes.String);
        o.ClaimActions.MapJsonKey(ClaimTypes.Email, "email", ClaimValueTypes.Email);
    });

And here is my controller:

public async Task<IActionResult> Index()
        {
            return View(await HttpContext.GetExternalProvidersAsync());
        }

        [HttpPost("~/signin")]
        public async Task<IActionResult> Index([FromForm] string provider)
        {
            // Note: the "provider" parameter corresponds to the external
            // authentication provider choosen by the user agent.
            if (string.IsNullOrWhiteSpace(provider))
            {
                return BadRequest();
            }

            if (!await HttpContext.IsProviderSupportedAsync(provider))
            {
                return BadRequest();
            }

            // Instruct the middleware corresponding to the requested external identity
            // provider to redirect the user agent to its own authorization endpoint.
            // Note: the authenticationScheme parameter must match the value configured in Startup.cs
            var challengeResult = Challenge(new AuthenticationProperties { RedirectUri = "/" }, provider);
            return challengeResult;
        }

        [Route("signin-discord")]
        public async Task<IActionResult> DiscordSignIn(string code)
        {
            Debug.WriteLine(code);            
            return RedirectToAction(nameof(LoginSuccess));
        }

Any ideas why it fails?