Troubleshooting plugins_url path issues with require_once in WordPress

Solution 1:
Try using plugin_dir_path( __FILE__ ):

class Maintenance {

    public function init()
    {
        add_action( 'wp_loaded', array( $this, 'maintenance_mode' ) );
        // add_action( 'admin_init', array( $this, 'maintenance_settings' ) );
    }

    public function maintenance_mode()
    {
        global $pagenow;

        if ( $pagenow !== 'wp-login.php' && ! current_user_can( 'manage_options' ) && ! is_admin() ) {
            header( $_SERVER['SERVER_PROTOCOL'] . ' 503 Service Temporarily Unavailable', true, 503 );
            header( 'Content-Type: text/html; charset=utf-8' );
            require_once plugin_dir_path( __FILE__ ) . 'assets/maintenance.php';
            die();
        }
    }
}

Solution 2:

Use:

require_once dirname( __FILE__ ) . '/assets/maintenance.php';

instead of:

require_once plugins_url( 'assets/maintenance.php', __FILE__ );