Bad news β good news.
The good one first: Easy FancyBox works, SlickrFlickr works, too. YES!
The bad one: WP Slimstats interferes with Easy FancyBox. I had to shut it off [ggrrrmmmmbbllll β f***].
The second bad one: I could have spared me a lot of work and sleepless nights, if I only had read this https://wordpress.org/plugins/easy-fancybox/other_notes/ carefully right to the bottom.
Now [because sometimes I have attacks of perfectionism] I going to try, if Responsive Lightbox has the same slimstats desease β¦
Dear Tom (and the others), many thanks for your support. As so often: PEBCAK *gg*