I kind of resolved the problem in a weird way.
1. I deactivated the fast boot of Windows 10
2. Reboot the system
3. Turn the PC off to "power save"-mode
4. Reactivate Windows
5. Turn on fast boot again
Now the CPU uses turbo boost whenever it's needed. Don't ask me why...^^ But it works.
The diagnostic tool shows me an error: IPDT64- Result: Fail
But based on the thread (CPU frequency fail) it's more an issue with the tool than my CPU.
I can't really rely on my warranty, because I opened the notebook case to change the HDD to a faster intel SSD...^^ As far as I know this voids the guarantee.