Crash with UnityAds

My Android users reports many of App crashes related to UnityAds. The crash occurs only approx with 1% of users. I can not reproduce the issue with my test devices. Has someone same problems and know the cause?

Here an example of crash report.
=========================================
java.lang.Error: FATAL EXCEPTION [main]
Unity version : 2018.3.2f1
Device model : LGE LG-H910
Device fingerprint: lge/elsa_att_us/elsa:8.0.0/OPR1.170623.032/1824715478c24:user/release-keys
Caused by
at com.unity3d.player.ReflectionHelper.nativeProxyInvoke (Native Method)
at com.unity3d.player.ReflectionHelper.a (Unknown Source)
at com.unity3d.player.ReflectionHelper$1.invoke (Unknown Source:6)
at java.lang.reflect.Proxy.invoke (Proxy.java:913)
at $Proxy8.onUnityAdsReady (Unknown Source)
at com.unity3d.ads.api.Listener$1.run (Listener.java:17)
at android.os.Handler.handleCallback (Handler.java:789)
at android.os.Handler.dispatchMessage (Handler.java:98)
at android.os.Looper.loop (Looper.java:164)
at android.app.ActivityThread.main (ActivityThread.java:6722)
at java.lang.reflect.Method.invoke (Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run (Zygote.java:240)
at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:770)[/code]

 

https://forum.unity.com/threads/cra...018-3-when-use-androidjavaproxy-class.627637/