Not returning to Flutter app after OAuth login

My Flutter app is currently not getting redirected back to my app once the OAuth login was successful.

I am using the Google OAuth login as followed:

await _account.createOAuth2Session(
  provider: OAuthProvider.google,
  scopes: [
    'https://www.googleapis.com/auth/userinfo.email',
    'https://www.googleapis.com/auth/userinfo.profile',
  ],
);

The strange thing is, that if I cancel the login, that the login in the app usually works fine. (A restart of the app alwasy works fine).

I have a self-hosted Appwrite backend with the following structure:
Appwrite: Version 1.5.5
Flutter SDK: 12.0.4
Hosted on: baas.example.com using digitalocean.com
Under Custom Domains, there is nothing listed. Trying to add anything does not work.

I am using nginx proxy manager to reverse proxy to baas.example.com and create its certificate.

Am I missing a necessary configuration? Do I have to add success and failure values? If so what are the values?

In the browser (web) I do get the following permanent redirect to: https://baas.example.com/auth/oauth2/success?project=*****&domain=.baas.example.com&key=a_session_630f8d82d13133b826cc&secret=***#

My website is hosting the app is using the domain app.example.com.