Clearing the cache did the trick - thank you Rich!!
PS: It is not clear if the “sudo openhab-cli stop” procedure mentioned in the “Clear the Cache”-thread (Clear the Cache - #101 by HaKuNa) is supported in OH3+:
sudo openhab-cli stop
[sudo] password for openhabian:
A systemd service configuration exists...
Use 'sudo /bin/systemctl stop openhab.service' to stop an openHAB service
Stopping any instance of openHAB...
Anyway, my system is up and running and I’m a happy Swede 