UnityAds Crash on Android

Unity Engine
,
1

Hey guys,
I am getting this error on some Android devices (e.g. Galaxy S8, Android 9):

Exception: No such proxy method: UnityEngine.Advertisements.Platform.Android.AndroidPlatform.onUnityAdsError(UnityEngine.AndroidJavaObject,System.String)
UnityEngine.AndroidJavaProxy.Invoke (System.String methodName, System.Object[] args) (at <00000000000000000000000000000000>:0)
UnityEngine._AndroidJNIHelper.InvokeJavaProxyMethod (UnityEngine.AndroidJavaProxy proxy, System.IntPtr jmethodName, System.IntPtr jargs) (at <00000000000000000000000000000000>:0)

I am using Unity 2018.4.14f1 and Unity Monetization Asset 3.4.1.
This is my ad-class:

using System.Collections;
using UnityEngine;
using UnityEngine.Advertisements;
using UnityEngine.Events;

public class AdManager : MonoBehaviour, IUnityAdsListener
{
    public void Initalize()
    {
        Advertisement.AddListener(this);
        Advertisement.Initialize(gameId, testMode);
    }

    public void ShowGameOverAd()
    {
        Advertisement.Show("video");
    }

    public void ShowExtraLifeAd()
    {
        Advertisement.Show("rewardedVideo");
    }

    public void OnUnityAdsReady(string placementId)
    {}

    public void OnUnityAdsDidError(string message)
    {
        Debug.Log("Error occured while trying to initialize Unity Ads: " + message);
    }

    public void OnUnityAdsDidStart(string placementId)
    {}

    public void OnUnityAdsDidFinish(string placementId, ShowResult showResult)
    {
        //my stuff
    }
}

Thanks for helping!

2

Probably best asked in the Ads forum Unity Services - Unity Discussions