I'm experiencing the same issues @thurlowjp is having posted here.
Whatever I add using VC [either on Backend or Frontend editor] it won't show up on the page.
Also, when I start a blank page and try to use VC Frontend, I have an error message when I click on the green "Add Text Block" button.
I'm working on a local site using Flywheel Local.
X Theme 5.2.3
WPBakery Visual Composer 5.2.1
All other plugins are deactivated.
I'll post credentials below.