WordPress Error “Not a Valid JSON Response”
The “Updating Failed. The response is not a Valid JSON Response” error from within the WordPress Guttenberg Editor could be the result of a number of issues. If you do some googling, there’s lots of information and ideas on how to rectify the error.
PHP Composer and Wrong PHP Version
In the case I had, the issue was related to a custom WordPress theme using composer and requiring PHP 8.x but the site infact operating on PHP 7.4
Simple fix, change the PHP version to PHP 8.x on the sites hosting control panel.