Some servers do not came with classic configurations, like server from one.com, where as default when a visitor goes to url, shows error 403 or show files list.
This happen due bad Apache .htaccess config, where do not have a rule to know wich file open by default when visitor come.
Add to .htaccess in web root folder or (sub)folder where you need to open index /specific file by default
Default standard config
DirectoryIndex index.html index.phpExample to open another file than index.php or index.html by default
Example to open maintance.html for any visitor by default. If you or any visitior add file name in the url, like domain.com/index.php, will access correctly to that file (if exist).
DirectoryIndex maintance.html