Fixing the 'Invalid JSON Response' Error in WordPress

If you've encountered the frustrating "Updating failed. Error message: The response is not a valid JSON response" error in your WordPress editor, you're not alone. This is a common issue, particularly for users of the block editor on local development sites or sites behind certain server configurations. This error prevents you from saving posts and pages, effectively halting your workflow.

This guide will walk you through the most common causes and their solutions, based on community troubleshooting.

Why This Error Happens

The WordPress block editor (Gutenberg) relies heavily on the WordPress REST API to communicate with your server. When you click "Update" or "Publish," the editor sends a request and expects a properly formatted JSON response back. If anything interrupts this communication or corrupts the response, you will see this error. Common culprits include:

• Permalink Structure: A misconfigured or corrupted permalink structure is a primary suspect.
• Plugin or Theme Conflict: A plugin or theme might be outputting unexpected content, breaking the JSON format.
• Server Configuration: Caching mechanisms (like AWS CloudFront) or security modules (like mod_security) can sometimes block or alter API requests.
• SSL/HTTPS Issues: A mismatch between your site's configured address and its actual URL can cause mixed-content errors.

How to Troubleshoot and Fix the Issue

Follow these steps in order, testing your editor after each one.

1. Reset Your Permalinks

This is often the fastest and most effective fix.

1. Navigate to Settings > Permalinks in your WordPress dashboard.
2. Simply click the "Save Changes" button without making any changes. This refreshes the rewrite rules stored in your database.
3. Test if you can now save a post or page.

2. Check for Conflicts

If resetting permalinks doesn't work, a plugin or theme is likely causing the problem.

1. Use the built-in Health Check & Troubleshooting plugin (often included in modern WordPress installs) to disable all plugins and switch to a default theme without affecting your live visitors.
2. If the error disappears in troubleshooting mode, reactivate your plugins and theme one by one, testing the editor after each activation, until you find the culprit.
3. As noted in one user's experience, the error may stop when using the Classic Editor plugin, which confirms the issue is specific to the block editor's interaction with your setup.

3. Review Your Site Health Status

WordPress's Site Health tool can provide valuable clues.

1. Go to Tools > Site Health.
2. Check the "Status" tab for critical recommendations. Pay close attention to any errors related to the REST API, as this is directly linked to the JSON error.
3. Address any critical issues listed there.

4. Investigate Server and Caching Configurations

For users on local development environments or behind services like AWS CloudFront:

• Local Environments: Ensure your local server stack (e.g., Local, XAMPP, MAMP) is up to date and correctly configured. Sometimes, a simple server restart can resolve the issue.
• Caching Services (e.g., CloudFront): If the error only occurs when going through a CDN, you must ensure critical WordPress paths are not being cached. As one user discovered, you must configure behaviors to exclude not just wp-admin/* but also API routes. Ensure paths like wp-json/* and index.php?rest_route=/* are set to bypass the cache entirely.

5. Verify Your Site Address

An inconsistency between your WordPress and Site URL addresses can cause this problem.

1. Go to Settings > General.
2. Ensure both the "WordPress Address (URL)" and "Site Address (URL)" are identical and use the correct protocol (http vs. https).

By methodically working through these steps, you should be able to identify and resolve the cause of the "Invalid JSON Response" error and get back to editing your content.