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If anyone happens to have a older version of the Aeotec Z-Stick Gen5 and wants to use it on a Raspberry Pi 4 will find out, that it will not work and at some point stumble over the GitHub thread about this issue: https://github.com/raspberrypi/linux/issues/3027

The fix to this issue is either to use a "dumb" USB-Hub (as discussed in the GitHub thread) or to mod the hardware of the stick. This mod was suggested by Liam in the Home Assistant forum.
It is as simple as rerouting resistor R33. All the details can be found in the Home Assistant forum thread

In any case... I happened to lose the tiny R33 resistor (rest in peace somewhere in my carpet little resistor) because I was distracted by a parcel delivery and had to come up with a fix... 

The tiny R33 resistor was according to Liam's post and my measurement 1.5 kΩ. I replaced the SMD resistor with two regular resistors (1 kΩ and 500 Ω) and the problem was solved.

The Aeotec Z-Stick Gen5 now works on my Raspberry Pi 4 as expected:

root@raspberrypi:~# lsusb
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 003: ID 0658:0200 Sigma Designs, Inc. Aeotec Z-Stick Gen5 (ZW090) - UZB
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub