A good point. I tested serveral versions. The problem is, that I cannot test much older versions becaus WhatsApp immediately says that the version is too old. So I cannot link the phone.
But: I realised that WhatsApp from their website runs ans links well when installed on older Android versions (I habe a phone using Android 6).
The Problem seems to hide in something WhatsApp tries to use in Android.
WhatApp became all rights on my phone, so that ist not an issue.