Dear all,
thank you for the intensive support and energy! It took me the night thinking about the problem: As the UI reacts on the item state shown with the gif images above, I was thinking: Maybe the icons are just not there (even though for temperature they are). As I reinstalled the latest openhabian freshly, I did a quick research and found this older issue.
With the following command from Kai in the karaf / openhab console the issue is fixed:
feature:install openhab-iconset-classic
Issue solved but I think a fix is needed => I’ll update the github issue.
Thanks anyhow - great support on finding and solving the problem!