@Stas
Just wanted to give you an update: It works.
I made a fresh install of domoticz and upon starting, it immediately got it's devices from the stick and made them available in the devices overview.

The only thing not working is the link quality indicator. It remains always at 0.

Ok, I'll post it tomorrow or wednesday, when I have some time. I am using the lights at the moment :) But I can see the messages from zigbee2mqtt / mosquitto in the domoticz log, so I don't think it is related to the mqtt server. Can you please post a few lines from your log - how it should look ...

Well, yes, sure. That's why I wrote that I don't see the paired device in the list.

I have installed zigbee2mqtt, mosquitto as well as the python plugin for domoticz. So far, everything looks good, I can pair devices and see messages in the log. However, I see no new device in the list. Do I have to make anything else to get my devices? I tried with an IKEA Tradfri 1000lm LED as ...