Plesk itself runs on a different webserver, so Nginx and Apache settings of the website should not affect the accessibility of the file for the built-in editor. What you could still do is to open your browser "Tools" console, like in Firefox for example "Extras" > "Tools" > "Tools for Web Developers" to find out more, e.g. if some specific error becomes visible when you try to edit the file through editor. Else I still think it is an ownership or permissions issue of the file or the path to that file.