Vladimir, I can provide an example.
I guess what Nicolas meant that all of the “sub-directories” has to be served by same script.
For example if you put your static web app (which is a Single Page React application with React Router) under folder /web/app you want all the sub folders, e.g. /web/app/subfolder to be served (routed) by /web/app. Currently /web/app/subfolder fails with 404 because such folder doesn’t exist.
Usually on apache web server it might be solved by applying .htaccess config, but it looks like Backendless doesn’t support it yet (according to Is there support for .htaccess files inside cloud hosting?).