WordPress Publishing Error

Problem

While working on a local WordPress project, I could not update or publish a post from the admin. WordPress sometimes returned the more useful message:

Publishing failed. Error message: The response is not a valid JSON response.

Solution

The JSON message initially suggested a Gutenberg or ACF problem. The actual issue was a protocol mismatch: the admin was loaded over HTTPS while the local site used HTTP, producing a CORS error.

Opening the local WordPress admin over HTTP allowed posts to publish normally.