I searched your forum but have not found a solution to my galleries (lightbox) that stopped working correctly. Photo Gallery Version 1.5.5 has worked properly for years but stopped opening lightbox upon clicking thumbnail. If you go to the bottom of the page and click next page then it starts working but visitors will not find this extra step.
Image click action
Open lightbox (selected option)
All other plugins were deactivated with no change in Photo Gallery.
Up to date WordPress 4.9.8, Elegant Themes 3.16.1 / Divi Child 1.0.1449762715
