@anlino
I think I need a new pair of glasses… – now it works fine.
Really annoying: The issue with the design shift is still there. If you compare draft page #1 with draft page #2 (with list-block and without additional CSS for the block) and live page, you will see a difference. This affects not only the frontend, but also the backend and may have nothing to do with your theme. I have no idea what triggers this behaviour. It was present (repeatedly) from a point in time that I can’t identify exactly.
Do you have any ideas? In any case, thank you very much for your help.
Hannes