Cause: change Wordpress's Permalink format. Eg. from "Plain" to "Post Name"

Solution for Wordpress Multi-site setup
WP root directory => delete ".htaccess"
My Sites => Network Admin => Settings => Network Setup => copy ".htaccess" content to WP root directory.

Single site:
WP root directory => delete ".htaccess"
Refresh Wordpress page to let http server regenerate new ".htaccess".

https://themeisle.com/blog/how-to-fix-updating-failed-the-response-is-not-a-valid-json-response/

https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-invalid-json-error-in-wordpress-beginners-guide/