This issue comes during uploading of einfachvorlesen version 1.0.15.

PlayStore printed the following error report:

We found some common issues that are preventing your app from being sent for review. You must address these issues before you can send your changes for review. Unspecified PendingIntent mutability Apps targeting Android 12 or later must specify the mutability of PendingIntents with either FLAG_IMMUTABLE or FLAG_MUTABLE. Having unspecified mutability may cause your app to crash.

Your app creates a PendingIntent without specifying whether it is mutable in the following places:

android.support.v4.media.session.MediaSessionCompat.<init>
androidx.media.session.MediaButtonReceiver.buildMediaButtonPendingIntent
com.google.android.gms.common.api.internal.GoogleApiManager.zaa
com.google.android.gms.common.api.internal.zas.zaaa
com.google.android.gms.measurement.internal.zzfj.zzeo
com.google.firebase.messaging.FirebaseMessaging.send

Update your app so that it specifies the mutability of PendingIntents.

See also : https://developer.android.com/about/versions/12/behavior-changes-12?hl=en#pending-intent-mutability

I think, it can be fixed with updates of all libs which are named above to their newest versions.