If you are using WordPress on a server with Imunify360, and get the error message “Updating failed. The response is not a valid JSON response.” check whether you have any 3rd party rule sets enabled. This has happened to me twice following an update, so I thought someone else might be helped by having a specific answer for the precise server setting.
To check, go to ModSecurity vendors on the WHM menu. It should list vendors and rule sets. In some cases, using any third-party rule sets will give trouble with WordPress.
I am also using CloudLinux, the LiteSpeed server, and LiteSpeed cache.