In case someone else runs into the same problem, I just wanted to report my recent experience:
I was setting up a second Home Assistant instance (for my cabin) on an RPi4 with the data partition on a USB3.0 SSD (the RPi4 booting from the SD card, but with the data partion on the SSD).
This worked well and is not the issue in this post.
Later I found that I had lost control of all zigbee devices on my home’s HA instance. My home’s HA instance is running on a NUC and zigbee uses the deconz integration and a ConbeeII stick connected via a USB extension cable.
After a couple of hours of despair, I found that the problem was that I had left my second HA instance (the one on the RPi4 with the USB3.0 SSD) on my desk close to the ConbeeII stick. USB3.0 is known (it wasn’t to me, but it is now) to create radio interference for 2.4 GHz devices.
Moving the RPi4 with the USB3.0 a meter or so away from the ConbeeII stick solved the problem and I regained control of my zigbee network.
As this was quite a surprise to me and since I could not find anything about this here at the forum, I thought I would mention this in case someone else experiences something similar.