Admin area: "Featured Image" link not opening in popup

Hi,

whenever I wanted to add a featured image to a post, the link would open a popup with options to choose from (upload, choose from library etc.).

Now the link opens within the same window, essentially closing the edit post page. When I click to add a picture as featured image, the page turns blank.

Any idea, what might have caused this and what I can do?

Lukas