Profile: lalK

Problem:
The REST API is one way that WordPress and other applications communicate with the server. For example, the block editor screen relies on the REST API to display and save your posts and pages.

Solution:
I checked that the customer's site has Cloudfare CDN on.

I found a similar case where the culprit of this issue was Cloudfare firewall blocking wp-json/wp/v2 requests.

Please check: https://toolset.com/forums/topic/updating-failed-the-response-is-not-a-valid-json-response-3/#post-2159331

I asked the customer to try to exclude wp-admin features from the firewall rules by following the instructions available here: https://developers.cloudflare.com/support/third-party-software/content-management-system-cms/improving-web-security-for-content-management-systems-like-wordpress/

Relevant Documentation:
https://developers.cloudflare.com/support/third-party-software/content-management-system-cms/improving-web-security-for-content-management-systems-like-wordpress/