Hi Andy,
Thanks again for taking the time to explain.
It took me all day but it seems that I was able to figure it out and to solve it.
It was a WPML related issue. As I had transferred my images to Amazon S3 and Imgix WPML had no clue where to find the Dutch Images. So every image url was wrong when viewing the site in Dutch. The background color is defined in the css and when no image is found to display the area is colored with the background color. See screenshot.
The plugin Media Cloud is not compatible (yet) with WPML officially but I found a workaround on their support site. After adding a tiny bit of code in my functions.php everything is working as expected.
Marked resolved!