Editor & Publishing Guides for WordPress

Updating Failed: The Response Is Not a Valid JSON Response (WordPress Fix)

You hit Update, the block editor turns red, and WordPress tells you the response is not a valid JSON response. The post is usually fine. What is broken is the REST API request between the editor and your server. Here is the order I work through it, from permalinks to SSL to the mod_security rule your host will not mention.

Aug 8, 2026