Dear @nikitaanisimov, many thanks for your reply. I have meanwhile found out that it had to do with the theme I was using. I have then switched to another theme and after a couple of other hick-ups, it’s working now.
https://jlerche.at/hochzeiten/
However, I have a different question now if I may. I am not entirely happy with the mobile view of the above page. The mosaic displays all pictures just one after the other in one column which ends up in a lot of scroll. Do you have any recommendation regarding settings, so that maybe at least 2 pictures are displayed next to each other?