Unity 5.5.4p4 with GPG for Unity crash on authentication

I'm using the latest personal Unity version, together with the latest version of GPG for Unity from Github. The moment I click Login or let the GPG login during start, my game crashes.

I've searching and tried so many things. What i've done so far:

• Downgraded the sdk tool to 25.2.3/.5

• Downgraded the plugins to play-services-*-10.0.1

• Different way of using the authentication part. So far nothing worked for me.

My leaderboard code is the following:

 using UnityEngine;
 using System.Collections;
 using GooglePlayGames;
 using UnityEngine.SocialPlatforms;
 using UnityEngine.UI;
 using GooglePlayGames.BasicApi;
 
 public class Leaderboard : MonoBehaviour
 {
     #region PUBLIC_VAR
     public string leaderboard;
     #endregion
 
     #region DEFAULT_UNITY_CALLBACKS
     void Awake()
     {
         Debug.Log ("!!! init");
         //PlayGamesPlatform.DebugLogEnabled = true;
         PlayGamesPlatform.Activate();
     }
     void Start ()
     {
         Debug.Log("!!! Login called");
         Social.localUser.Authenticate((bool success) =>
         {
             if (success)
             {
                 Debug.Log("Login Sucess");
             }
             else
             {
                 Debug.Log("Login failed");
             }
         });
     }

Does anyone have a suggestion what to do next? My game is available in Google Play Store under closed Alpha (with my email/account added). I'm testing on my own device.

I've tried so many things and already lost more than a day to get this to work.