Image uploader is displaced

Hi everyone!

I’m trying to add images to a post, and when I click on the link to upload pictures, the overlay appears at the bottom, and only a 1/4 is visible, so I can’t upload any images.
I’ve tried several browsers and it’s always the same.

Here’s a screenshot. You can see how I can barely see the overlay, and that it appears at the bottom of the page, so I need to scroll down to see it (otherwise I would think it’s not opening at all).

Thanks in advance for your help!