Thank you so much. I changed the ownership of the direcotry as you said and it worked.
So my steps were:
$ sudo systemctl stop zigbee2mqtt
$ sudo systemctl start zigbee2mqtt
$ sudo journalctl -u zigbee2mqtt.service -f
I got the Error Message: “Error: EACCES: permission denied, mkdir ‘/var/log/zigbee2mqtt’”
I created the folder myself with
$ sudo mkdir /var/log/zigbee2mqtt’
Then changed the ownership of the directory as @Wolfgang_S suggested with:
$ chown openhabian:openhab /var/log/zigbee2mqtt
Now Zigbee2MQTT runs again.
Hope this helps anybody who has the same problem after a reboot.