In Debug mode, Google Play Games shows it's loading screen and I can even log in. However, afterwards I am receiving this error in a clean message box:
The application is incorrectly configured. Check that the package name and signing certificate match the client ID created in Developer Console. Also, if the application is not yet published, check that the account you are trying to sign in with is listed as a tester account. See logs for more information.
In Release mode, the game crashes and I am receiving this error in Android Studio:
Access Not Configured. The API (Google Play Game Services API) is not enabled for your project. Please use the Google Developers Console to update your configuration.
In Google Play Developer Console, all the marks are ticked. I don't understand why it is not working. Should I do something with the SHA key or client ID?
I did not publish my settings yet, but it says The Google Play game services settings are ready to test. I use my developer account as test account on the same device as I am testing on.
I was using the wrong SHA1 keys. I had to create a new GPGS Game and perform step 3C on this page: https://developers.google.com/games/services/console/enabling