[Q] Win10 fastboot - Doesn't work. Once worked. S1boot doesn't show up device manager

Hi,

I tried every way and guides shown here. Even disabling driver integrity checks et cetera.

Currently, there's a default stock ROM on my Xperia Z2.
I install the adb-setup-1.4.2.exe installer which installs adb / fastboot + the drivers for my X64 Win10.

Alright.
Reboot.

Then I plug in the USB cord into my turned off phone and hold Volume Up.
My phone vibrates.
I release the button.

Then in tthe Device Manager, this shows up:

Everytime.the.same.****ing.THING!

Anybody had the same problem?
One time, through some curious wonder, it worked. S1boot showed up and even the correct driver was instaled (back then via the adb-setup by a XDA user).
Well, what am I doing wrong?

I would be glad for any help. I've tried any troubleshooting guide there is by now and I am at the end of my knowledge.

Thanks for any help,
Michael