This guide walks WordPress site owners, developers, and hosting support staff through diagnosing and fixing a “valid JSON response” error.
- Applies to: WordPress 6.x (Full‑Site Editing enabled)
- Error text:
Updating failed. The response is not a valid JSON response.
Quick Summary of What Breaks
- WordPress sends a REST API request when you click Save on a template part:
POST /wp-json/wp/v2/template-parts/<ID> - Server returns something that is NOT valid JSON (empty, HTML, 403, 500, etc.).
- Site Editor shows the generic error pop‑up.
Because the failure is server‑side, no amount of client‑side refreshing will fix it—you must identify and remove the server‑level blocker introduced during migration.
Symptoms You May See
| Symptom | Details |
|---|---|
| Error modal | “Updating failed. The response is not a valid JSON response.” |
| Plugins ruled out | Disabling all plugins does not change the outcome. |
| Front‑end unaffected | The live site renders fine; only the Site Editor save fails. |
| Started after migration | Site worked on staging; broke the moment it went live. |
Baseline Checks (Already Performed)
These are common culprits that have already been ruled out. Repeat them if you are working from scratch.
- Plugin conflict – deactivate all plugins.
- Site Health – Tools → Site Health shows no critical errors related to REST API.
- URL consistency – Settings → General → both WordPress Address & Site Address use https and identical domain.
- Permalinks flush – Settings → Permalinks → Save Changes (regenerates
.htaccess). - Cache purge – clear any server / plugin / CDN cache layers.
If the error still occurs, continue to the next section.
Step‑by‑Step Deep‑Dive
Tip: Perform each step, then retest saving. Stop once the error disappears.
1 Collect Server Error Logs (Most Insightful)
Ask your host (e.g., GoDaddy) for the PHP error log entries corresponding to the exact time you click Save. Provide them with a timestamp (include timezone). Look for:
- PHP fatal errors or warnings in
/wp-json/requests mod_security/ WAF rules blocking REST endpoints- Permission‑denied or missing‑file messages
2 Enable Local WP Debug Logging
If you can edit wp-config.php, add above the line that reads / That’s all, stop editing / :
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );Reproduce the failure, then download and inspect /wp-content/debug.log.
3 Verify REST API Endpoint Directly
Run (or ask your host to run): curl -I https://your‑domain.com/wp-json/wp/v2/
A healthy site returns HTTP 200 and JSON headers. A 403/500 implies a server or firewall block.
4 Check File & Directory Permissions
Migration tools occasionally tighten permissions too far.
- Directories: 755
- Files: 644 Ensure
/wp-content/themes/and/wp-content/uploads/follow those rules.
5 Compare .htaccess (or nginx config)
Look for rules that inadvertently block /wp-json/ or rewrite it. If on Apache, temporarily rename .htaccess; if the WordPress saves, a rule inside is the culprit.
6 Temporarily Disable Security Modules
Many hosts run mod_security or a Web Application Firewall (WAF).
- Ask the host to toggle off mod_security for your domain and retest.
- If the error disappears, whitelist the offending rule or adjust the firewall.
7 Database Integrity Check (Rare, but Possible)
Template parts live in wp_posts with post_type = 'wp_template_part'.
- Locate the row.
If corrupted, deleting it allows WordPress to recreate the template and can clear the error.
7. Remove all emojis and special symbols
In some hosts you may experience the error if you used emojis or special symbols.
Escalation Script for Your Hosting Provider
Copy‑paste the following into your support chat/ticket:
We migrated our WordPress block‑theme site on [DATE]. Immediately afterward, only the Site Editors template fails to save with the error: “Updating failed. The response is not a valid JSON response.” Plugin conflicts, permalink flush, and Site Health are all clear. Please check the server’s PHP error logs and any WAF/mod_security rules that might block
POST /wp-json/wp/v2/template-parts/*. Also confirm file permissions (uploads/themes) and that the REST API is reachable. The issue did not exist before the migration, so we suspect a server‑level block or misconfiguration.
Preventing a Repeat During Future Migrations
| Checklist Item | Why it helps |
| Use an up‑to‑date PHP version (≥ 8.1) | Older versions have REST bugs fixed in later releases. |
| Migrate via a plugin (e.g., Duplicator) or SSH rsync, not through host “clone” buttons | Ensures complete DB search‑and‑replace and preserves permissions. |
| Verify REST API endpoints before and after DNS cutover | Catches JSON issues early. |
| Keep server logs for 48 h after go‑live | Allows quick rollback or targeted fixes. |
Additional Resources
- WordPress Handbook: REST API Error Reference
- Core ticket #52527 – Generic JSON response error lacks context (why the message is so vague)
- GoDaddy Help: View Error Logs in cPanel
Still Stuck?
If you have followed every step and the WordPress still will not save and your host can’t identify the blocked request, consider:
- Temporarily switching to a default block theme (Twenty Twenty‑Four) to test if the issue is theme‑specific.
- Engaging a WordPress developer to inspect the server in real‑time.
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