Critical error when trying to access Appearance / Customize

General
1

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All the themes and plugins are up to date and the site is running fine, as well as the rest of the Wordpress backend which also runs just fine.

When I change the theme the problem persists.

Can anyone help me?

2

The error message you’re seeing — “There has been a critical error on this website” — is a generic WordPress error that typically means something is wrong at the PHP level, even if the site otherwise seems to be functioning. Since:

  • The rest of the WordPress admin works fine,
  • All plugins and themes are up to date,
  • The error persists even when changing the theme,

The issue is likely tied to either:

  • A custom function or modification in functions.php,
  • A conflict triggered when loading the Theme Customizer (which the error URL suggests),
  • PHP memory limits or other server-level restrictions.

Here’s how to systematically diagnose and resolve it:

1. Enable WordPress Debug Mode

Edit your wp-config.php file and add or change the following lines:

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

Then reproduce the error and check the log at /wp-content/debug.log.

2. Manually Check Theme’s functions.php

Sometimes minor PHP syntax errors or undefined functions in functions.php can cause critical errors — even with default themes.

Steps:

  • Use FTP or your host’s file manager.
  • Navigate to /wp-content/themes/your-active-theme/functions.php.
  • Temporarily comment out any recent custom code.

3. Temporarily Rename mu-plugins (If Present)

If you’re using Must-Use Plugins (in /wp-content/mu-plugins), try renaming the folder temporarily to disable them. These aren’t affected when changing themes or toggling regular plugins.

4. Check PHP Version Compatibility

Make sure your host is running a PHP version compatible with your WordPress and theme/plugins. WordPress 6.x prefers PHP 8.0+ but some older plugins/themes can break with PHP 8.1 or 8.2.

5. Use Health Check Plugin

If you can access other admin areas, install the Health Check & Troubleshooting plugin:

  • Enable “Troubleshooting Mode”
  • Activate the default theme and one plugin at a time to isolate the issue.

Would you like help analyzing the debug.log file if you enable it and upload it here? That would provide very specific clues. The error message you’re seeing — “There has been a critical error on this website” — is a generic WordPress error that typically means something is wrong at the PHP level, even if the site otherwise seems to be functioning. Since:

  • The rest of the WordPress admin works fine,
  • All plugins and themes are up to date,
  • The error persists even when changing the theme,

The issue is likely tied to either:

  • A custom function or modification in functions.php,
  • A conflict triggered when loading the Theme Customizer (which the error URL suggests),
  • PHP memory limits or other server-level restrictions.

Here’s how to systematically diagnose and resolve it:

1. Enable WordPress Debug Mode

Edit your wp-config.php file and add or change the following lines:

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

Then reproduce the error and check the log at /wp-content/debug.log.

2. Manually Check Theme’s functions.php

Sometimes minor PHP syntax errors or undefined functions in functions.php can cause critical errors — even with default themes.

Steps:

  • Use FTP or your host’s file manager.
  • Navigate to /wp-content/themes/your-active-theme/functions.php.
  • Temporarily comment out any recent custom code.

3. Temporarily Rename mu-plugins (If Present)

If you’re using Must-Use Plugins (in /wp-content/mu-plugins), try renaming the folder temporarily to disable them. These aren’t affected when changing themes or toggling regular plugins.

4. Check PHP Version Compatibility

Make sure your host is running a PHP version compatible with your WordPress and theme/plugins. WordPress 6.x prefers PHP 8.0+ but some older plugins/themes can break with PHP 8.1 or 8.2.

5. Use Health Check Plugin

If you can access other admin areas, install the Health Check & Troubleshooting plugin:

  • Enable “Troubleshooting Mode”
  • Activate the default theme and one plugin at a time to isolate the issue.

Would you like help analyzing the debug.log file if you enable it and upload it here? That would provide very specific clues.