Hello Gerhard,
The problem is solved. After looking at the SSLLabs results again i noticed a difference between the Android 7.0 handhake. Our site gave ECDH secp384r1 while your site gave ECDH secp256r1. After some googling i landed on the following site:
https://stackoverflow.com/questions/39133437/sslhandshakeexception-handshake-failed-on-android-n-7-0
So there seems to be a bug in Android 7.0. I now changed the line “ssl_ecdh_curve secp384r1” to “ssl_ecdh_curve prime256v1” in “web.ssl.template.yml” and after a rebuild the app started working.