I found the problem. A very good friend of mine had the idea to use the ANR Watchdog from SalomonBrys to get the logs I was missing to find the problem. Check it out https://github.com/SalomonBrys/ANR-WatchDog.

The WatchDog revealed that an infinity loop existed in an old part of the App. That loop created preasure on the main thread. After removing the loop everything works as expected and I can sleep again.

Thanks to everyone for suggesting and putting your thoughts into this.