Post add/edit error

Hi, if I activate the plugin and then try to edit or create a new post, the featured image won’t load or if I add a new gallery the same thing happens. And when I save the post, the gallery is not saved.

The error in the console when loading and saving is:

data.min.js:2 Uncaught (in promise) {code: "invalid_json", message: "The response is not a valid JSON response."}