Right, thanks for the hint, Boban. I downloaded my keystore when I created and published my first app and saved it locally. Since then I create almost all apps and releases with this keystore.
So if you have not saved the valid keystore, see here:
http://www.geekcodehub.com/2018/05/23/keystore-lost-in-android/
https://stackoverflow.com/questions/41105053/android-how-to-recover-keystore-file
https://forums.xamarin.com/discussion/134593/how-to-recover-lost-keystore