Code-Auth returning invalid_grant (Blazor Server)

[Silvan1009]

Hey there, I am currently playing with the api. Everything runs fine when i use ResponseType.Token for the LoginRequest. But for my future project i need code authorization. When i copy together the code from the examples in the documentation i get the error response "{"error":"invalid_grant","error_description":"Invalid authorization code"}". My granted code starts with "AQBe7POyLUpgMgMHAnh" is 232 characters long and contains several hyphens and underscores. At least the format seems right to me. I really dont know why the api grants a authorization code if the contained information is wrong. I double checked the parameter parsing. The literal from the url lands in my code correctly and is passed to the api completely. Am i missing a step? Here is my code. It runs within a razor-page in a blazor server app.

    protected override async Task OnInitializedAsync()
    {
        var query = HttpUtility.ParseQueryString(new Uri(_navManager.Uri).Query);
        var code = query.Get("code");

        if (code == null)
        {
            var loginRequest = new LoginRequest(_redirectUri, _clientId, LoginRequest.ResponseType.Code)
                {
                    Scope = new[] { Scopes.PlaylistReadPrivate, Scopes.PlaylistReadCollaborative }
                };

            _authUri = loginRequest.ToUri();

            return;
        }

        var response = await new OAuthClient().RequestToken(new AuthorizationCodeTokenRequest(_clientId, _clientSecret, code, _redirectUri));
        _spotify = new SpotifyClient(response.AccessToken);
    }

I don't know if I am encountering a bug or if I am on a wrong path. My Project is set to .net8 and the API is on Version 7.2.1.

Hopefully someone can help me. I would be very grateful