Meanwhile, I came across this thread (https://dietpi.com/forum/t/mosquitto-failed-on-v7-8-2-with-rpi-b/5936/1) in the forum that describes a similar problem. Is there a way to roll back the version of mosquitto to 2.0.11 as described here (https://github.com/eclipse/mosquitto/issues/2384#issuecomment-978347768) by an RPI Zero user?
I tried for now to install this via apt-get install mosquitto=2.0.11-1 libmosquitto1=2.0.11-1, which seems to work:
root@rpi-smarthome:~# dietpi-services restart mosquitto
DietPi-Services
─────────────────────────────────────────────────────
Mode: restart mosquitto
[ OK ] DietPi-Services | restart : mosquitto
root@rpi-smarthome:~# dietpi-services status mosquitto
DietPi-Services
─────────────────────────────────────────────────────
Mode: status mosquitto
[ OK ] DietPi-Services | mosquitto active (running) since Tue 2021-11-30 13:28:36 GMT; 12s ago