JSON Error: “Updating failed. The response is not a valid JSON response.”

“Updating failed. The response is not a valid JSON response.”

I receive this error under a couple circumstances. 1) I cannot use the WordPress default Editor to add gallery or slider blocks as the error appears and I cannot save the Post. 2) If I use Elementor and add Gallery/Slider this produces a proper Post (see link). However, I still cannot use default WP editor to add a Feature Image, or schedule the Publish, it throws the error.

The page I need help with: [log in to see the link]