nadwordpress on "[Plugin: WP Maintenance Mode] Internal server error"

Hi

I have:
- WordPress 3.9
- Windows, IIS7, URL Rewrite, Web.config modified
- PHP 5.2.17 with FASTGCI enabled

Just installed wp maintenance mode 1.8.11. Once activiated, I get:
"The page cannot be displayed because an internal server error has occurred."

Thanks in advance for your support.

https://wordpress.org/plugins/wp-maintenance-mode/