Missing images in thumbnail preview

When previewing the thumbnails of an image from the media library on a site in a subfolder (example.com/wp/), the path of the second image in every pair starts with a /, which makes it relative to the top of the domain, instead of the site.

The WordPress convention is to always use absolute URLs based on site settings. Wrapping those URLs in home_url( RELATIVE_PATH ) should do the trick and product the correct result (assuming the uploads folder is also correct).