Marypop on "[Plugin: Forget About Shortcode Buttons] 500 Internal Server Error"

I use WP 4.3.1 and I installed FASC 1.1.0.

When I click toolbar button, I got message:

500 Internal Server Error
The request was not completed. The server met an unexpected condition.
xxx.xxx.xxx/wp-content/plugins/forget-about-shortcode-buttons/assets/js/fasc-buttons/popup.php?ver=1.1.0&source=insert&ajaxurl=http%3A%2F%2Fxxx.xxx.xxx%2Fwp-admin%2Fadmin-ajax.php (port 80)

Any idea why?
Earlier I've been using this plugin and it has worked well.

Thanks!

https://wordpress.org/plugins/forget-about-shortcode-buttons/