Install extension “maintenance” over extension manager.
Configure “adminOnly” setting during installation (in extension manager). This setting will be set automatically if maintenance mode will be activated. After deactivation, the old value (from Install-Tool) will be set back.
Check the Install-Tool and configure the following parts:
Check the Report module for configuration mistakes and correct them
After this, you can activate / deactivate maintenance mode with only one click from the toolbar (upper right corner, next to clear cache)