Thumbnails are missing on attachments page

I recently have updated a large WP website (core and plug-ins). Now, all thumbnails are missing when I display the attachments page of a PDF document.

When I look at the source code of that page, the image file name is missing at the end of the image path, for example:

https://www.example.com/wp-content/uploads/2015/08/
instead of
https://www.example.com/wp-content/uploads/2015/08/some-pdf-document-thumb.png

This happens only on the attachment page. When I insert the same document as a document gallery into a post, everything is fine. And the thumbnails exists!