I am the main user/access of this site.

I updated our plugins and themes. Everything was successful, though only ONE page of the entire website has hit a 'critical error'.

It appears to have lost margining and displays not images like the other pages. The posts within the pages are okay.

We use Cornerstone for the editing. I have noticed that the pages that are not working use HTML as their Editing option. This has always been the case; though why have they not carried across, perfectly, like the other pages following the update?

The developer told me why he saved the page like this.

"I didn't set up the Portfolio pages in Cornerstone because they're meant to automatically gather the posts from a particular category and display them. Cornerstone doesn't have this ability, unless you do it manually each time you create a new row of posts (Cornerstone only allows one row at a time)."

Changing the page to Cornerstone also does not work. It reverts to HTML. This page's user interface is no different to the other pages, so the setting should not be different.

Would it be a 'Site-ground' thing to solve, perhaps?

Child Theme X is our page theme. Also updated.