The PHP engine attempts to read a file but the operating system blocks the request. The block occurs because the file or its directory does not grant read or execute rights to the user that runs the web server. It also happens when the file belongs to a different user or group, when PHP settings restrict the accessible paths, or when security policies such as SELinux intervene.
The warning appears in the WordPress front end, admin dashboard, or during plugin and theme operations. It signals that the code cannot reach a required file, which stops execution and may produce a blank screen.