Hi Jonathan, there are several things going wrong here at the same time:
First: there are two Uncaught TypeError errors occurring. These prevent Easy FancyBox from even working. What you are seeing is the effect of the theme fancybox script and the plugin script combined. The errors that need to be fixed are coming from the surbma-smooth-scroll.js script (which seems to think jQuery is available under $ which in WordPress it isn’t) and from the form maker plugin main_div_front_end.js which seems to be (forced to?) use an alternative jQuery library that should NOT be there in the first place: <!-- Dieser Quellcode wurde hinzugefügt von WP Analytify (2.1.13) !-->https://code.jquery.com/jquery-latest.js
These errors need to be addressed before Easy FancyBox will start working as it was intended at all.
Next: there is other FancyBox code in the theme that might interfere. Not sure though, as first the errors need to be fixed. But it might turn out to still not work as expected because of the theme integrated code…
Last: go to your Setting > Media page and change the Section(s) field value from “div.content, div.gallery” to “div.wpb_wrapper” (without the quotes)