popup window doesnt close properly

[JeyKJey]

When I click on a button for a picture, the window with a picture pops up BUT I cannot close it, the close button doesnt work.
When the window pops up, the background of the website turns dark grey and in order to close the window, I have to click on the background, it turns light grey and only then I can close the popup window by clicking on the close button.

What can be the problem? Thanks in advance