This was a really strange issue.
I have about 18 network sites on my multisite installation, all running the same theme (Parabola).
Since yesterday 1 of my sites seems to have suddenly stopped rendering/loading correctly in CHROME (lots of black squares which only reveal after hover over, or the actual post text doesn't load).
Here's a screenshot of what I got:
The same issue occured on Chrome for ios.
The site loaded ok in Internet Explorer, just not Chrome.
I removed all plugins and the site was working then started tracking down to the precise plugin and found it was this one.
No idea why it would cause an issue on just 1 site (especially when it's not even being used on this one)