There are multiple pages where I use the gallery. Once I click on an image and open it and then hover it with my mouse there’s a large darkgrey area covering the page.
The problem doesn’t show up on every computer, but on about 95%.
It looks like you are experiencing a plugin conflict. I can see that flexslider is being called in by the theme possibly and ReFlex Gallery uses flexslider. This would mean that the code is being called in twice. I’m not sure if you can remove the themes version and retest.