Ok, now it's getting strange.
I reinstalled the HTC driver again from the hasoon2000 website. And now, my M8 appears
under the available storage devices. But only when I disable the USB debugging! If debugging is enabled still nothing happens!
As a last try I have installed the "Universal ADB driver" again... didn't change anything.
I really don't know what's going wrong.
The M8 works on my Win7 notebook, my Win7 PC at work and it worked well on my develop PC (in a peaceful coexistence with my Samsung Galaxy Nexus), before I installed Windows8.1 (fresh install, no upgrade!)