Safer mu-plugins call

In your example under troubleshooting, it throws a fatal error if the plugin isn’t present. You should change the code to the following:

if (file_exists(WP_PLUGIN_DIR . '/wp-native-php-sessions/pantheon-sessions.php')) {
    require_once WP_PLUGIN_DIR . '/wp-native-php-sessions/pantheon-sessions.php';
}