You click Save or Publish and the block editor says “Updating failed,” often followed by “The response is not a valid JSON response.” Your changes are not saved. The block editor saves through the WordPress REST API, so this error means something is blocking or breaking that request. Here are the causes we see most, in the order we check them.
1. Check Site Health
Go to Tools → Site Health. If it reports “The REST API encountered an error,” that confirms the problem and often names the cause, such as a 403 or 401 response.
2. Re-save permalinks
Go to Settings → Permalinks and click Save Changes without changing anything. This rebuilds the rewrite rules. It fixes the error more often than you would expect, especially after a migration.
3. Security plugin or firewall rules
Security plugins, Cloudflare or a host firewall (ModSecurity) sometimes block REST API requests as suspicious, especially when the content contains code, iframes or script-like text. Temporarily disable the security plugin or firewall rule and try again. If saving works, whitelist /wp-json/ requests for logged-in users rather than leaving protection off.
4. Mixed content or a URL mismatch
If Settings → General shows the WordPress Address and Site Address with http:// while the site loads over https:// (or with and without www), REST requests can fail. Make both addresses match exactly how the site loads. See fixing mixed content errors.
5. A plugin conflict
A plugin that outputs extra text or a PHP warning into the API response breaks the JSON. Use Health Check troubleshooting mode to test without affecting visitors: our plugin conflict guide walks through it.
6. Look at the actual response
Open your browser developer tools, go to the Network tab, and click Update again. Click the failed request (usually to /wp-json/wp/v2/posts/...) and look at the Response. A PHP warning, an HTML error page or a firewall block page tells you exactly where to look.
7. Server limits
Very long posts can exceed PHP’s post_max_size or a server request size limit. If only your longest page fails to save, ask your host to raise those limits.
Avoid: switching to the Classic Editor
Installing the Classic Editor plugin hides the error but does not fix the REST API, which other plugins and features depend on. Fix the cause instead.
Still failing?
Send it to us. The $79 one-time fix covers it, fixed or your money back. A Care Plan keeps issues like this from showing up after updates.