An (old) friend of mine has a website (Apache/2.4.10 (Ubuntu) Server) since many years. He provide, in several folders, several PDF files (research papers from he). He is yet very old (over 80) and probably has made a mistake.

Since few days, in one of these folders, the content is entirely blank, instead of displaying the usual "Index of /".

In all other folders, the website display the content correctly, e.g.:

But in this particular folder, it's blank (no "Index of /", no "Apache/2.4.10 (Ubuntu) Server", nothing).

What might have happened? The files in the blank folder are still accessibles with the complete path (e.g. at "http://www.someWebSite.com/folder1/subfolder/paper.pdf" if "subfolder" is the blank folder).

I don't have great knowledge in websites, Apache and so on, but I like helping this friend. Is there a missing configuration file? Bad permissions? He uses the software FileZilla for managing his website. What can or must be controlled?