I have disabled .opcache, though it does appear that the issue persist and requires a constant clearing of the /wp-content/cache folder as well. This is most likely caused by the following functionality :

Code:
/wp-content/themes/g5_helium/custom/css-compiled/helium_products.css?ver=59c90b4c


As you can see in the example above, the theme is trying to generate a compiled CSS under :

Code:
g5_helium/custom/css-compiled


Which it does, however the application when trying to load that file calls it from cache instead of the actual location of the file :

Code:
wp-content/themes/g5_helium/custom/css-compiled/helium_products.css


in the /public_html/ of the account. As soon as the /wp-content/cache is cleared, the page comes up correctly with the proper CSS

This leads me to believe that the way that Gantry is generating the cache is conflicting with the way that your theme is calling it's CSS.

You may want to double-check the Gantry configuration on the matter, as it doesn't appear that there is an option to disable Caching within Gantry's settings at all. There is an option to flush the cache, but nothing other than that from what I found.

Unfortunately, you need root access in order to be able to disable mod_security rule for a website and this is why you need to open new Support Ticket with detailed step-by-step guide how to replicate the issue on our end and we will disable the needed rule only for the the website in question.