to give you a little update about the current state of my work, here is what I tried already.
First, I was not able to solve the problem. KabelDeutschland (Vodafone Kabel) changed something in their API and within their Apps.
Normally I would use the same strategy I did to start this project in the first place, reverse engineering.
Unfortunately KabelDeutschland(Vodafone Kabel) now scans on every startup of the App (Android and iOS) if something listens to the connection. I am not able to use burp as a proxy to read the HTTPS encrypted traffic by now. It always tells me that I am not connected to the internet.
I will have to find another way to get into the encrypted communication of the app, to find out what they changed within their API.
If anyone has an idea, I would love to hear it.