Google Play Service - LogIn Prompt not showing up

Hi Guys

I've tried to implement the GPGS into an existing Application. The app is uploaded to the Google Play Store and the status is set to Alpha Test. I have imported the Minimal Demo, which is trying to sign in a user.

When I execute the function:

 Social.localUser.Authenticate((bool success) =>
 {
      mWaitingForAuth = false;
      if (success)
      {
            mStatusText = "Welcome " + Social.localUser.userName;
      }
      else {
           mStatusText = "Authentication failed.";
      }
 });

no Login Prompt is showing up, neither in the Unity Editor or on my Android Phone.

In the Start() function, I activate the PlayGamesPlattform:

 GooglePlayGames.PlayGamesPlatform.Activate();

The logcat gives following error:

PopupManager: You have not specified a View to use as content view for popups. Falling back to the Activity content view. Note that this may not work as expected in multi-screen environments

GamesNativeSDK: Auth operation started: SIGN IN

GamesNativeSDK: Connecting to Google Play...

GamesServiceBroker: Client connected with SDK 10084000, Services 10084436, and Games 37230036

PopupManager: No content view usable to display popups. Popups will not be displayed in response to this client's calls. Use setViewForPopups() to set your content view.

PopupManager: No content view usable to display popups. Popups will not be displayed in response to this client's calls. Use setViewForPopups() to set your content view.

GamesServiceBroker: Client connected with SDK 8487000, Services 10084436, and Games 37230036

PopupManager: No content view usable to display popups. Popups will not be displayed in response to this client's calls. Use setViewForPopups() to set your content view.

GamesServiceBroker: Client connected with SDK 8487000, Services 10084436, and Games 37230036

GamesNativeSDK: Play Games callback indicates connection failure.

GamesNativeSDK: UI interaction required to connect to Google Play.

GamesServiceBroker: Client connected with SDK 8487000, Services 10084436, and Games 37230036

My Android Configurations are looking like:

Directory to save con...: Assets

Constants class name: Minimal.GPGSIds

My Ressources are defined and my Web App Client ID is also set.

In the Developer Console, the Game is also created in the Services and linked to my existing Google Play Store Project. The Entry is also publicated and there are no errors.

I hope somebody has an idea what cases these errors.

Thank you for answers! :)