'fastboot devices' returns ??? (question marks) and device reboots

I came across this weird issue and wanted to share my findings somewhere. At some point in the installation process of Magisk , I was required to use fastboot on Windows 10 21H2. I proceeded and downloaded the adb plattform-tools + adb drivers . Previously ADB-related things of the instruction (like adb-shell) worked flawlessly. However during the usage of fastboot I encountered these issues: I tried different USB-cables, -ports and -drivers.. When I ran fastboot devices (or any other fastboot-command, it returned ???????????? (or an error) and the device restarted.. I was pretty certain that the device itself was fine, as well as the installation of ADB tools and driver. Nothing seemed to work and I only read about solutions like using a different PC, which was no option in my opinion.