The response should be visible in the network tab of your browser’s developer tools. Have network tool open before doing whatever triggers the error message. Click on the related API request in the log list. In the sidebar that appears, go to the Response tab.
TBH, I’m unsure if seeing the response will be enlightening. Usually the problem is not that the response is malformed, the problem is the API itself is not working correctly. While the problem could be that a theme or plugin has broken the API, it’s usually a server configuration issue such as firewall or modSecurity not allowing WP to make HTTPS requests to itself. That said, it’s not a bad idea to check the response anyway since it’s easily done once you know where to look.
Thank you for your help. Most appreciated.