Hello Sandro,
It's done! After a lot of searching and trying, I have finally found the culprit. As you rightly suspected, it is hiding in the task scheduling library. It is "Mcupdate_scheduled", which can be found under Microsoft / Windows / Mediacenter. After unchecking the "Reactivate computer to run the task" box, the PC will no longer wake up unintentionally.
You have helped me a lot. Thank you.
Best regards
Gerd