Hi Bruno,
I have tested by disabling your W3 Total cache minifications and also by disabling W3 Total cache but your issue persist. Most probably, it could be that your X theme or Cornerstone plugin may have been corrupted.
You can delete your X theme and Cornerstone plugin, and then re-install it again by downloading the latest version from your Themeforest account. Then you can use a plugin like Advanced Reset (https://wordpress.org/plugins/advanced-wp-reset/) to start over (this plugin will wipe all your current data and do not use this if you want to keep your current data).
Now you also need to install and activate Revolution Slider plugin through X -> Validation section before you generate your demo content. Also install install the Contact Form 7 plugin as well.
Finally before generating your demo content, make sure to disable other plugins as well (specially the caching plugin) and then try generating your demo content and see if that helps.
Thanks!