Fixing the 'Error parsing the response to JSON' in Yoast SEO

If you've encountered the frustrating "Error parsing the response to JSON" message while trying to run Yoast SEO's data optimization, you're not alone. This is a common issue that can prevent the plugin from properly scanning your content and building its internal linking and SEO data. This guide will walk you through the most effective solutions, based on community reports and successful fixes.

What Causes This Error?

This error typically indicates a communication problem between the Yoast SEO plugin and the WordPress REST API. The optimization process relies on this API to function. When the API returns an unexpected response—often an HTML error page instead of clean JSON data—the process fails. Common causes include:

• Conflicts with other plugins or your theme.
• Server-level security rules (like mod_security) or firewalls (e.g., Wordfence) blocking API requests.
• Corrupted data from a previous optimization attempt.
• An outdated version of Yoast SEO Premium running alongside the free plugin.

How to Fix the "Error parsing the response to JSON"

Try these solutions in order, starting with the most common fix.

Solution 1: Reset Yoast SEO's Data

The first and most successful step is to reset the plugin's internal data tables using the Yoast Test Helper plugin. This clears any corrupted data that might be causing the optimization process to fail.

1. Install and activate the free Yoast Test Helper plugin.
2. Go to Tools > Yoast Test.
3. In the Yoast SEO section, click the following buttons one by one. The page will reload after each click:
   • Reset indexables tables & migrations
   • Reset Prominent words calculation
   • Reset Internal link counter
4. Once finished, go to SEO > Tools.
5. Under "SEO data," click the Start SEO data optimization button.

Solution 2: Check for Plugin or Theme Conflicts

If resetting the data doesn't work, a conflict with another plugin or your theme is likely. The best way to check for this is by using the Health Check & Troubleshooting plugin.

1. Install and activate the Health Check & Troubleshooting plugin.
2. Go to Tools > Site Health > Troubleshooting and enable troubleshooting mode. This will deactivate plugins for your user session only, leaving the site normal for visitors.
3. While in troubleshooting mode, activate only Yoast SEO and your theme (preferably a default theme like Twenty Twenty-Four).
4. Try running the SEO data optimization again. If it works, you have a conflict.
5. Re-enable your plugins one by one, testing the optimization after each one, to identify the culprit. Common conflicting plugins include security tools (e.g., Wordfence), caching plugins (e.g., LiteSpeed Cache), and page builders (e.g., Elementor).

Solution 3: Check Your Browser's Console for Errors

Technical errors can provide clues. While on the SEO > Tools page, open your browser's developer console to see if any JavaScript errors are reported.

1. Right-click on the page and select Inspect.
2. Click on the Console tab.
3. Try starting the SEO data optimization again.
4. Look for any red error messages in the console and note what they say. These messages can be helpful for further troubleshooting.

Solution 4: Investigate Server and Security Settings

Sometimes, the issue is not with WordPress but with the server configuration. The error message can provide a hint.

• 403 Forbidden Error: This points to a security rule or firewall blocking the request. As seen in one thread, temporarily disabling a security plugin like Wordfence resolved the issue. Check your security plugin's settings for rules that might block WordPress REST API requests and whitelist them if necessary.
• 500 Internal Server Error: This is a generic server error. Contact your web hosting provider and ask them to check your server's error logs for any specific messages related to the /wp-json/yoast/v1/ API endpoint. They can also check for issues with mod_security or other server-level firewalls.

Solution 5: Ensure Yoast SEO Premium is Updated or Removed

If you have an old, inactive copy of Yoast SEO Premium installed alongside the free plugin, it can cause significant conflicts and errors.

1. Go to Plugins > Installed Plugins.
2. If you see "Yoast SEO Premium" and do not have an active subscription, deactivate and delete it.
3. You should only have the free "Yoast SEO" plugin active.
4. After removal, use the Yoast Test Helper to reset the indexables again and retry the optimization.

When to Seek Further Help

If you have tried all these steps and the error persists, the problem may require more advanced debugging. The community on the WordPress support forums can often help if you provide details like:

• The full error message.
• A screenshot of your browser's console errors.
• Any relevant error logs from your hosting provider.

Remember, this error is almost always solvable by methodically working through conflicts and server configurations.