Pictures from library don't show in post editor media upload screen

Hi there,
On this website (https://loewenhardtfoundation.org/2web/) I’m working on a funny thing is happening. The most recent pictures uploaded in the media library don’t show in post editors’ media upload screen. They show in the Media Library list though. The upload of a picture in the Editor works allright, but the thumbnail disappears from the insert screen when adding another picture.

The problem occurred a few days ago, before it was working fine. I’m not using any caching plugins. The major plugin installed is WPML (CMS and String Translation). I’m using a child theme with the Genesis framework.

Any ideas someone?
Thanks.