Can't set 'extra image'

Hello,

when I click on ‘set featuring image’ it’s not a problem.
I can click on the button, the pop up appears in the middle of the screen and I can select my image. The WordPress page itself turns dark (on the background) and the pop up is bright as normally the whole page would be.

But, when I click on ‘set extra image’, the pop up appears but the whole screen becomes dark. So, when I click on anywhere in the pop up, it closes.

Working from a macbook pro the problem occurs in safari as well as in Chrome.

Anyone?