thanks for helping me. Its totally strange. I tried everything the last days and i couldnt confirm, that the issue is related to an USB device. I unplugged all stuff and it didnt help.
Today i´ve got a new Windows 10 Update and installed that. I re-booted and the issue was still there.
But i disconnected all kinds of USB devices again to test and this time i could verify, that the Thrustmaster HOTAS Warthog Throttle was obviously the reason.
I could leave pedals and the joystick connected, but the Throttle seems to prevent the PC from going to activate it´s screensaver.
I checked the RAW Input Values coming from that device but i couldnt nail it down to a specific button or anything.
I contacted the Thrustmaster Support. Hopefully they can create a new Firmware or Driver to prevent this. My solution for now is to disable the Throttle via the power buttons on my active powered USB Hub.