A PHP session was created by a session_start() function call. This interferes with REST API and loopback requests. The session should be closed by session_write_close() before making any HTTP requests.

This error occurs because the session_start() function is creating a PHP session, which interferes with REST API and loopback requests in WordPress. To resolve this:

1. Locate the session_start() code
   Search your theme or active plugins for the session_start() function. It may be in the theme’s functions.php file or in a custom plugin.
2. Add session_write_close() after session_start()
   Ensure session_write_close() is called after each session_start() call, before making any HTTP requests. This will close the session and allow the REST API to work correctly. For example:
   session_start();
// Your code here
session_write_close();

3. Use the init hook in WordPress
   If session_start() needs to be in functions.php, make sure to wrap it in the init hook and use session_write_close() like this:
   add_action('init', function() {
if (!session_id()) {
session_start();
session_write_close();
}
});