How to Fix “Updating Failed. The Response Is Not a Valid JSON Response”

If you use WordPress regularly, you may have encountered the frustrating message: “Updating failed. The response is not a valid JSON response.” It commonly appears when you try to update or publish a post or page using the WordPress Block Editor. Instead of saving your changes, WordPress displays the error and leaves you wondering whether the problem is with your content, plugin, theme, hosting, or WordPress configuration.

The good news is that this error is usually fixable. The message generally indicates that WordPress expected a valid JSON response from its REST API, but the server returned something different—such as an HTML error page, a 403/404 response, a PHP warning, a security block, or another unexpected response.

WordPress support discussions show that plugin conflicts, REST API problems, server security rules, HTTP/HTTPS configuration, and server-side errors can all contribute to invalid JSON responses.

In this guide, we’ll explain what the error means, why it happens, and the most effective ways to troubleshoot and fix it.

What Does “The Response Is Not a Valid JSON Response” Mean?

JSON stands for JavaScript Object Notation. WordPress uses JSON extensively to exchange information between the browser and the server through its REST API.

When you click Update or Publish in the Block Editor, WordPress sends a request to the server. The server is expected to return a structured response that WordPress can understand.

If the expected JSON response is replaced by something else, WordPress cannot process the request and displays:

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

For example, a server might return a 403 Forbidden page, a PHP warning, a firewall message, or an HTML document instead of the JSON data the editor expects. WordPress community troubleshooting discussions specifically recommend checking the actual REST/API response because the visible error message alone does not identify the underlying cause.

This means the error is often not a problem with JSON itself. Instead, something is interfering with the request or modifying the server’s response.

Common Causes of the Invalid JSON Response Error

There is no single cause for this WordPress error. Several different configuration problems can produce the same message.

1. WordPress REST API Problems

The WordPress Block Editor relies heavily on the REST API. If REST API requests are blocked, redirected incorrectly, or returning unexpected data, posts and pages may fail to save.

A REST API problem can result from:

• Server configuration
• Security software
• Firewall rules
• Plugin conflicts
• Incorrect URL configuration
• Authentication or permission problems
• Web server rules

Therefore, checking whether your REST API is functioning correctly should be one of the first troubleshooting steps.

2. Incorrect WordPress URL Settings

An incorrect WordPress Address (URL) or Site Address (URL) can interfere with requests between your browser and WordPress.

For example, your website may use HTTPS publicly while WordPress is configured with HTTP, or your preferred domain may use www while another version is configured internally.

Check:

WordPress Dashboard → Settings → General

Review:

• WordPress Address (URL)
• Site Address (URL)

Make sure both use the correct protocol and domain.

For an HTTPS website, you would normally expect the URLs to use:

https://

rather than:

http://

Also check whether your website is consistently using either the www or non-www version according to your preferred canonical configuration.

Avoid making URL changes blindly on a production website. Incorrect URL settings can create redirects, login problems, and additional site issues.

3. SSL or HTTPS Configuration Problems

SSL configuration is another potential cause.

Suppose your website loads over HTTPS, but an API request attempts to communicate through HTTP. The request may be redirected, blocked, or handled differently by the server.

This can become especially complicated when your website uses:

• SSL certificates
• Cloudflare
• CDN services
• Reverse proxies
• Security plugins
• Server-level redirects

Make sure your SSL certificate is valid and that your WordPress installation consistently uses HTTPS.

You should also check whether browser developer tools show failed REST API requests or redirect loops.

4. Plugin Conflicts

A plugin can interfere with WordPress REST API requests and cause the Block Editor to receive an unexpected response.

Possible plugin categories include:

• Security plugins
• Firewall plugins
• Caching plugins
• Optimization plugins
• SEO plugins
• Custom API plugins
• Content management plugins
• Authentication plugins

WordPress support discussions have documented cases where plugin conflicts were associated with invalid JSON responses.

How to Test for a Plugin Conflict

Before changing anything permanently, create a backup or use a staging website if possible.

Then:

1. Temporarily deactivate your plugins.
2. Open the affected post or page.
3. Make a small change.
4. Click Update.
5. Check whether the error disappears.

If the problem disappears, reactivate the plugins one at a time.

After activating each plugin, test the editor again.

When the error returns, you may have identified the plugin causing the conflict.

Do not automatically assume that the last plugin you installed is responsible. An update to an existing plugin can also introduce a compatibility problem.

5. Theme Conflicts

Themes can also affect WordPress functionality.

A theme may contain custom PHP code, filters, REST API modifications, or other functionality that changes how WordPress processes requests.

To test this possibility, temporarily switch to a default WordPress theme.

If the problem disappears after changing themes, investigate your active theme or child theme.

This is particularly important when your website contains:

• Custom functions
• Child-theme modifications
• Custom REST API code
• Custom AJAX functionality
• Security snippets
• Deprecated PHP code

Always test carefully before changing the theme on a live business website.

6. Firewall or ModSecurity Blocking the Request

One of the most important causes to investigate is server-level security.

Web hosting environments may use security systems such as:

• ModSecurity
• Web Application Firewalls
• Server security rules
• Hosting firewalls
• CDN security systems

These systems can sometimes interpret a legitimate WordPress REST API request as suspicious and block it.

In that situation, WordPress may receive an HTML error page or a 403 response instead of JSON.

WordPress support discussions include examples where server security rules or ModSecurity were suspected or identified as the source of blocked requests.

How to Check

Open your browser’s developer tools:

Right-click → Inspect → Network

Then try to update the post again.

Look for a failed request, particularly one associated with:

/wp-json/

or a WordPress REST API endpoint.

Click the failed request and examine the Response section.

If you see:

• 403 Forbidden
• 404 Not Found
• 500 Internal Server Error
• HTML instead of JSON
• Security/firewall messages

you have a strong indication that the problem is happening outside the editor itself.

Contact your hosting provider and ask them to review the relevant server logs and security rules.

7. Regenerate WordPress Permalinks

Incorrect or outdated rewrite rules can sometimes cause REST API requests to behave unexpectedly.

A simple troubleshooting step is to regenerate your permalink settings.

Go to:

Dashboard → Settings → Permalinks

You do not necessarily need to change your permalink structure.

Simply review the current structure and click:

Save Changes

WordPress will refresh its rewrite rules.

Afterward, return to your post and test the update again.

If your website uses custom .htaccess rules, server-specific rewrites, or advanced redirects, check those rules carefully as well.

8. Caching and Optimization Problems

Caching systems can occasionally interfere with dynamic WordPress requests.

This may happen when a caching or optimization system incorrectly caches an API request or modifies JavaScript behavior.

Potential sources include:

• WordPress caching plugins
• Server-side caching
• CDN caching
• Browser caching
• JavaScript optimization
• HTML optimization
• API caching

As a troubleshooting step, temporarily disable caching or optimization features and test the editor.

If the problem disappears, re-enable features individually until you identify the problematic setting.

Be particularly careful with aggressive JavaScript optimization because the WordPress editor depends on JavaScript to communicate with the REST API.

9. PHP Errors or Unexpected Output

A PHP warning, notice, error, or custom output can corrupt a response that should contain only JSON.

For example, if a plugin outputs an error message before WordPress generates its REST API response, the resulting response may no longer be valid JSON.

This is why checking your site’s error logs can be extremely valuable.

Look for:

• PHP fatal errors
• PHP warnings
• Deprecated-function messages
• Plugin errors
• Theme errors
• Memory-related errors

WordPress support troubleshooting has also recommended debugging and checking PHP errors when invalid JSON responses occur.

If you have access to WordPress debugging tools, enable debugging appropriately on a staging or development environment rather than displaying errors publicly on a production website.

10. Check Browser Developer Tools

If the basic solutions do not work, inspect the actual request.

This is one of the most useful methods because the WordPress error message is generic.

Step-by-Step

1. Open the affected post.
2. Press F12 or right-click and select Inspect.
3. Open the Network tab.
4. Click Update in WordPress.
5. Look for a failed request.
6. Open the request.
7. Review the Status Code.
8. Check the Response tab.
9. Review the request URL and response content.

The status code can provide a major clue.

Common Status Codes

200 – The server accepted the request, but the response may still contain unexpected output.

301/302 – A redirect may be interfering with the API request.

400 – The request may be malformed.

401/403 – Authentication, permissions, firewall, or security rules may be involved.

404 – The requested REST API endpoint may not be available.

500 – A server-side or PHP error is likely.

This approach is more reliable than repeatedly installing troubleshooting plugins without identifying the actual failed request.

How to Fix the Error: Recommended Troubleshooting Order

If you want to solve the problem efficiently, follow this order:

Step 1: Save Your Content

Before troubleshooting, copy important content or create a backup.

Step 2: Check WordPress URLs

Go to:

Settings → General

Confirm your WordPress and Site URLs are correct.

Step 3: Verify HTTPS

Make sure your SSL certificate works correctly and your site uses the correct HTTPS configuration.

Step 4: Refresh Permalinks

Go to:

Settings → Permalinks → Save Changes

Then test again.

Step 5: Clear Caches

Clear:

• WordPress cache
• Server cache
• CDN cache
• Browser cache

Then reload the editor.

Step 6: Test Plugins

Temporarily disable plugins and test the update.

Step 7: Test the Theme

Temporarily switch to a default theme if necessary.

Step 8: Inspect the REST API Request

Use browser developer tools to identify the failed request and response.

Step 9: Check Server Logs

Look for PHP, firewall, ModSecurity, or web-server errors.

Step 10: Contact Your Hosting Provider

If the response is being blocked at the server level, your hosting provider may need to investigate firewall or server configuration.

What If the Error Happens Only on One Post?

If other posts update correctly but one particular post produces the error, the problem may be related to that content rather than the entire WordPress installation.

Try these tests:

• Remove recently added blocks.
• Remove custom HTML.
• Remove unusual embeds.
• Remove recently added shortcodes.
• Copy the content into a new draft.
• Test the post without recently added media.
• Check whether a particular custom field or block causes the problem.

There are WordPress support reports where specific content, blocks, or plugin functionality were associated with the error.

If the error occurs on every post and page, investigate the REST API, server, plugins, theme, URL configuration, and security layers first.

Should You Reinstall WordPress?

Usually, no—not as your first step.

Reinstalling WordPress may replace core files, but it will not automatically fix:

• Firewall rules
• ModSecurity configuration
• Plugin conflicts
• Theme code
• Incorrect URLs
• CDN configuration
• SSL problems
• Server permissions
• PHP errors

Identify the underlying cause first.

A WordPress core reinstall may be considered later if you have evidence that core files are damaged or incomplete, but it should not be the default solution for an invalid JSON response.

How to Prevent WordPress JSON Response Errors

Once you fix the problem, take steps to reduce the possibility of it returning.

Keep WordPress Updated

Use supported and compatible versions of WordPress, plugins, and themes.

Avoid Untrusted Plugins

Install plugins from reputable sources and keep the number of unnecessary plugins low.

Test Major Changes

Use a staging website when making significant changes to:

• PHP
• Themes
• Plugins
• CDN settings
• Firewall rules
• Caching
• Server configuration

Monitor Server Errors

Regularly review server and PHP logs if you manage your own hosting environment.

Keep SSL and URLs Consistent

Make sure your domain, HTTPS configuration, redirects, and WordPress URL settings work together correctly.

Be Careful With Security Rules

Security is important, but overly aggressive firewall rules can interfere with legitimate WordPress requests.