editing width of container for tablet/mob - error message

Hi there
I'm creating a site with a container and a grid with three columns inside it.
I want the container inside each of the three columns to be a different % width of the screen on desktop ( 33, 33, 33) table (50, 50, 75) and mobile (100, 100, 100).
However when I click on the setting for 'tablet' or 'mobile' I get an error message and the whole edit screen vanishes.
Here's the error text (not v informative):
The editor has encountered an unexpected error.

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I've never had this problem before.
I had siteground's caching / optimising plugin installed but not configured, and have now deactivated it in case that's relevant.
Site is in development and usually behind an under construction page but have opened it for the next few hours so you can check it.
Thank you
Liz