No fancybox effect since WP5 introduction

I just found that if I switch off Speed Booster Pack plugin, fancybox effect comes back. I am not an expert but I read that Speed Booster does some changes to increase page loading speed. It “eliminates external render-blocking javascript and css in above-the-fold content, moves scripts to the footer to improve page loading speed, minifies HTML and JavaScript to increase your page load speed, loads javascript files from Google Libraries rather than serving them from your WordPress install directly, defers parsing of javascript files to reduce the initial load time of your page.”

I want to have both plugins but for the time being one excludes the other one.

Hello,
unfortunately optimization plugins can merge javascripts in the wrong order, creating unexpected behaviours.

If Booster Pack allows it with some advanced option, you can try forcing the original working order of the javascripts so the correct behaviour is preserved.
Also, you could try other optimization plugins – I never used Booster Pack but I have used another (Autoptimize by Frank Goossens) and it did work correctly with the builtin lightbox.
Right now I have activated it on https://simplestgallery.com enabling JS/CSS/HTML optimization and with this simple operation I instantly produced an improvement on Google Pagespeed from score 35 to 44.

Sorry for late reply, hope it can help!