@ubeaut, thanks for your answers.
Since I’m new to Zigbee, I had decided to upgrade from OH3 to OH4 before starting with it. That gave me another headache (Memory usage with Openhabian OH4, Raspberry Pi 3 Model B), which I’ve now resolved by installing the 32-bit OS.
Sometimes I get the message “No supported clusters found” as reported here: OH3: ZigBeeThingHandler] - No supported clusters found (70 down lights down the drain?!) - #12 by chris, so I tried OH 4.1.0-M3 and then the latest snapshot, hoping that Update ZSS library to 1.4.12 by cdjackson · Pull Request #809 · openhab/org.openhab.binding.zigbee · GitHub would solve my problem, but no luck.
I checked the ownership of /dev/tty*0 and they’re both with the group dialout. I also tried without the other USB device and get the same behaviour. Since I’m getting a lot of zigbee messages in the log (I changed the log level to debug) and they all seem okay (to my untrained zigbee eye), I don’t think there’s actually a problem with the Sonoff. This is further supported by the fact that some other zigbee devices correctly registered and return data. I just happened to start with one of the device types that doesn’t work.
I think I’m at the point where I try zigbee2mqtt and see if it works better, but that’ll have to wait now until I have a little more time.
Thanks again for your help 