Images doesn't show in sites/default/files

So, I was uploading a couple of pictures to my (newly built Drupal 8) site. When uploaded the images didn’t show, but (depending on which browser I use) the alt-text or an outline of the place where the image would be was shown.

All images were put in a directory within sites/default/files. The permissions of the images (and also of sites/default/files) are set on 755.
If I drag and drop the images to the sites/default directory, the images will show. Apparently something is causing trouble in the sites/default/files directory.

What is wrong with my sites/default/files directory so images won’t show? And what can I do to fix the problem?

Drupal version: