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Troubleshooting Critical Errors and 500 Issues with Custom Post Type UI

22 threads Sep 16, 2025 Plugin → Custom post type ui

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Encountering a critical error or a blank white screen when managing your custom posts is a common and frustrating issue for users of the Custom Post Type UI plugin. Based on community reports, these problems often manifest as:

  • A generic 'There has been a critical error on this website' message.
  • A white screen of death when adding or editing posts.
  • Admin pages loading without styles, accompanied by console errors related to admin-ajax.php.
  • Error 500 (Internal Server Error) messages.
  • 'Updating failed. The response is not a valid JSON response' errors.

These symptoms are typically not caused by a fundamental flaw in the Custom Post Type UI plugin itself, but rather by conflicts or server-side issues that surface on the admin screens it generates. The core of the problem is almost always a PHP error that is being hidden by your site's default error reporting settings.

How to Find the Root Cause: Check Your Error Logs

The single most important step in resolving these issues is to identify the specific PHP error causing the crash. Generic 'critical error' messages are unhelpful; you need the details. Here’s how to find them:

  1. Server Error Logs: The most reliable source. Access your web hosting control panel (e.g., cPanel, Plesk) and look for sections labeled 'Error Logs', 'Logs', or 'Metrics'. The logs here will contain detailed PHP Fatal Errors, Warnings, and Notices that pinpoint the exact file and line number causing the problem.
  2. WordPress Debug Log: Enable WordPress's built-in debugging system to create a detailed log file on your server. To do this, edit your site's wp-config.php file and ensure the following lines are present and set to 'true': 
    define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true ); // Logs errors to wp-content/debug.log
define( 'WP_DEBUG_DISPLAY', false ); // Prevents errors from being printed on-screen
    After reproducing the error, check the wp-content/debug.log file for entries. Remember to disable WP_DEBUG and WP_DEBUG_DISPLAY after troubleshooting for security and performance.
  3. Admin Email: WordPress may automatically send the details of a fatal error to the site administrator's email address. Check the inbox for the email associated with your WordPress installation.

Common Causes and Their Solutions

Once you have the specific error message, you can apply a targeted fix. Common causes include:

  • Plugin or Theme Conflicts: This is the most frequent culprit. A function in your theme or another plugin may be incompatible with the WordPress core, PHP version, or another plugin. Solution: Temporarily switch to a default WordPress theme (like Twenty Twenty-Four) and disable all other plugins except Custom Post Type UI. If the error disappears, reactivate your theme and plugins one by one to identify the conflict.
  • Outdated Code/Deprecated Functions: If your site or a plugin has not been updated, it might use old PHP functions that are no longer supported in newer PHP versions. Solution: The error log will often name the problematic function. Update all plugins, themes, and WordPress core. If the error persists, contact the developer of the outdated plugin or theme.
  • Incorrect Server Configuration: Issues like ModSecurity rules blocking requests or incorrect permissions can cause 500 errors. Solution: The error log may mention 'ModSecurity' or 'permissions'. Contact your web hosting provider's support team and provide them with the specific error log entry; they can often adjust server settings or whitelist rules.
  • Corrupted Plugin Files: In rare cases, files may not have uploaded completely or correctly during an update. Solution: As one user found, manually reinstalling the plugin via FTP can resolve the issue. Delete the custom-post-type-ui folder from wp-content/plugins/ and upload a fresh copy.

Specific Scenario: The 'Not a Valid JSON Response' Error

This error typically indicates a problem with the AJAX request used by the block editor (Gutenberg). While it can be related to conflicts, it's also frequently caused by code in your theme's functions.php file or a plugin that is outputting unwanted characters (like blank spaces) before or after PHP code, corrupting the JSON response. Carefully check any custom code for errant whitespace outside of PHP tags.

By systematically enabling debugging, checking logs, and isolating conflicts, you can move past the vague 'critical error' and efficiently resolve the underlying problem with your custom post types.

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