You click Edit in Gutenberg and the page instantly crashes with: There has been a critical error on this website. This usually means a fatal PHP error or REST API failure triggered by a plugin, theme function, or server setting. Gutenberg is very JavaScript-heavy and interacts with the REST API, so anything that blocks it causes a crash.

Below are the proven fixes in order of impact.

1. Enable Debug to See the Real Error

You can’t fix what you can’t see.

Edit wp-config.php:

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

Reload the Gutenberg editor.
The actual error will appear in:

/wp-content/debug.log

Typical real-world causes:

• Fatal error in plugin
• PHP type errors (PHP 8.x strict)
• Missing class from a theme
• API conflict

2. Disable Performance/Optimization Plugins

Gutenberg must load dozens of JS modules. Cache or optimization plugins break it.

Temporarily disable:

• LiteSpeed Cache
• WP Rocket
• Hummingbird
• Autoptimize
• Asset CleanUp
• FlyingPress
• Nitropack

If Gutenberg loads normally, the conflict is confirmed.

Then:

• Turn off JS combine/minify
• Disable defer/async for admin
• Exclude /wp-admin/, /wp-json/, block-editor*

3. Check REST API Health (This is the #1 Root Cause)

Gutenberg works via REST API endpoints.
If REST is blocked, editor crashes.

Check:
Tools → Site Health → Status

Look for:
“The REST API encountered an error”

Reasons include:

• Security plugins blocking /wp-json/
• Cloudflare firewall rules
• ModSecurity blocking POST requests
• Incorrect .htaccess rules

Fix:

• Allow /wp-json/*
• Whitelist your domain in firewall
• Disable “Block REST API” options

4. Update Outdated Plugins or Block Plugins

A single block extension can break the editor.

Disable plugins one by one:

• Kadence blocks
• Spectra (Ultimate Addons)
• Elementor blocks
• Gutenberg add-ons
• WooCommerce blocks

If the editor opens, that plugin is the culprit.
Update or replace it.

5. Fix PHP Version Issues

PHP 8.2 / 8.3 introduces stricter errors like:

• Cannot use string offset
• Deprecated dynamic properties
• Missing typed return

If your theme/plugin is old, it crashes instantly.

Try:

• Switch to PHP 8.1 temporarily
• Update plugins/themes

Gutenberg core works on PHP 8.x. Third-party code may not.

6. Increase PHP Memory Limit

Low memory stops Gutenberg mid-load.

In wp-config.php:

define('WP_MEMORY_LIMIT', '512M');
define('WP_MAX_MEMORY_LIMIT', '512M');

Restart the editor.
For shared hosting, set this in your panel if wp-config.php is ignored.

7. Clear Browser, CDN, and Server Cache

Old JS chunks = instant crash.

Do:

• Purge CDN (Cloudflare, Bunny, LiteSpeed CDN)
• Clear hosting cache
• Open Gutenberg in incognito

If it works in private mode but not normal mode → browser cache issue.

8. Theme Issues

Bad theme functions can kill the editor.

Test with:
Appearance → Themes → Activate Twenty Twenty-Four

If Gutenberg starts working, your theme has:

• Outdated blocks
• Fatal errors in functions.php
• Breaking enqueue scripts

Check:

• Custom JS/CSS
• Block overrides
• Old child theme code

9. Server-Level Blocking

Some servers kill Gutenberg due to security rules.

Examples:

• Nginx or Apache limits
• ModSecurity
• Firewall POST body filters

If editing crashes only on large posts:

• Increase client_max_body_size (Nginx)
• Increase LimitRequestBody (Apache)

10. Last Resort: Reinstall Core

Won’t delete your content.

Dashboard → Updates → Reinstall WordPress

This repairs corrupted Gutenberg JS bundles.

Quick Checklist

• Turn on debug → read debug.log
• Check Site Health → REST API status
• Disable optimization plugins
• Update block plugins + theme
• Increase PHP memory to 512M
• Try PHP 8.1 if 8.3 breaks plugins
• Switch to default theme
• Purge CDN + security rules

Once the offending plugin, stale cache, or REST restriction is removed, Gutenberg loads normally again.