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.