On a project with several folder 

(Note : there is no problem with a project containing few folders)

• Choose a file and create private link
• Open list view
• From the Files module (address ending in documents.html), copy the file's private link into the address bar.
  => It opens correctly in a pop-up window.
• Click the X to close the pop-up window => the address ends in documents.html again.
• Copy the file's private link into the address bar again => Nothing happens, no pop-up window.