can't click edit popup

Hi,
if I try to edit a picture of a gallery some popup shows up. The problem is, I am not able to click at any button becouse there is a slight grey area over the whole window. ESC = no action, ENTER = popup closes.

Has somebody an idea what’s going wrong there?

Thank you, greetings

http://wordpress.org/plugins/nextcellent-gallery-nextgen-legacy/