Recently I ran into a particular error on the production version of prufcreative.com:
At first this issue seemed like a new version of the dreaded WordPress white screen of death.
However, it turns out there’s apparently a server-level rule on WPEngine sites, with Global Edge Security enabled, that prevents some Gutenberg blocks from being saved, resulting in the “Updating failed. The response is not a valid JSON response.” error message.
I was advised specifically by a typically cheerful and helpful WPEngine support rep that the best course of action is to contact them ahead of time (with FSE themed sites) and “requesting to allow
wp-json requests in GES per a domain basis.”