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Troubleshooting Common Force Regenerate Thumbnails Errors

48 threads Sep 17, 2025 Plugin → Force regenerate thumbnails

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If you're using the Force Regenerate Thumbnails plugin and have encountered cryptic error messages or unexpected behavior, you're not alone. This guide compiles the most common issues reported by users and provides steps to diagnose and resolve them.

1. The "function (){...}" or "Undefined" Error

This is one of the most frequent and confusing errors. Instead of a success or failure message, you see a block of minified JavaScript code or simply the word "Undefined."

Why it happens: This is almost always caused by PHP warnings or notices being output before the plugin's JSON response. These warnings corrupt the data, causing the browser to display the raw code. Common triggers include:

  • Having WP_DEBUG set to true in your wp-config.php file.
  • Permission issues on your server's uploads directory (wp-content/uploads/).
  • Conflicts with other plugins that output errors.

How to fix it:

  1. Disable WP_DEBUG: Open your wp-config.php file and ensure the following line is set to false: define( 'WP_DEBUG', false );.
  2. Check File Permissions: Using your FTP client or hosting file manager, ensure the wp-content/uploads/ folder and all subfolders have correct write permissions (typically 755 for folders and 644 for files). In some cases, users have temporarily set permissions to 777 for troubleshooting, though this is not recommended for production long-term.
  3. Enable Debug Logging: To find the underlying error without breaking the plugin, enable WordPress debugging to a log file. In wp-config.php, set: 
    define( 'WP_DEBUG', true );
define( 'WP_DEBUG_DISPLAY', false );
define( 'WP_DEBUG_LOG', true );
    This will create a debug.log file in your wp-content directory. Reproduce the error, then check the log for any PHP warnings or notices that need to be resolved.
  4. Conflict Test: Temporarily deactivate all other plugins and switch to a default WordPress theme (like Twenty Twenty-One). If the error disappears, reactivate your plugins one by one to identify the one causing the conflict.

2. "The originally uploaded image file cannot be found"

The plugin fails because it cannot locate the original image file on your server.

Why it happens:

  • The file was manually moved or deleted.
  • You are using a CDN (Content Delivery Network) or cloud storage plugin (like Cloudinary or Offload Media) that has moved the original files off your server. The Force Regenerate Thumbnails plugin requires the original file to be physically present on your server's disk to work.
  • There is a server configuration issue causing the file_exists() PHP function to fail, even though the file is present.

How to fix it:

  1. Check File Existence: Use FTP to verify the original image file actually exists at the path shown in the error message.
  2. Disable CDN/Offload Plugins: If you use a plugin to offload media to the cloud, you will likely need to temporarily disable it and sync your files back to your local server before regenerating thumbnails. The plugin is not compatible with remotely stored originals.
  3. Path Issues: Some users have reported that certain server configurations require modifying the plugin's code to correctly parse the file path. This is an advanced solution and should be approached with caution, preferably by a developer.

3. "Failed: ... is an invalid image ID" or cURL Error 22

Regeneration fails for specific images with an invalid ID message or a cURL error.

Why it happens: The invalid image ID error can occur if the image record in the WordPress database is corrupted. The cURL error 22 often relates to an HTTP request failing, which can be caused by an incorrect file path or a server-level issue.

How to fix it:

  1. Re-upload Problem Images: For the images that fail, the simplest fix is often to delete them from your media library and re-upload them.
  2. Check Server Resources: The cURL error could indicate a timeout. If you have a large number of images, try regenerating them in smaller batches.

4. Files with the Same Name But Different Extensions

If you have two files with the same name but different extensions (e.g., photo.jpg and photo.png) in the same upload folder, regenerating one will delete the thumbnails of the other.

Why it happens: This is a known limitation of the plugin. It uses the base filename (without the extension) to find and delete old thumbnails, which causes a conflict between files that share a name.

How to fix it: The only reliable solution is to rename one of the files so they no longer share the same base name before regenerating thumbnails.

5. 503 Errors and Extreme Server Slowness

Your site becomes unavailable or extremely slow during or after regeneration.

Why it happens: Regenerating thumbnails is a very resource-intensive process (CPU and memory). If you have many large images, the process can exceed your hosting plan's resource limits, causing the server to throttle or temporarily shut down your site.

How to fix it:

  1. Process in Batches: Never regenerate your entire library at once if you have more than a few hundred images. Use the bulk action tool on smaller selections of images.
  2. Check Hosting Resources: Consider upgrading your hosting plan if you consistently hit resource limits. A VPS or dedicated server environment is better suited for heavy processing tasks.
  3. Run During Off-Peak Hours: Schedule the task for a time when your site has the least traffic.

General Best Practices

  • Always Backup: Before running any bulk operation on your site, ensure you have a complete backup of your database and wp-content directory.
  • Keep Updated: Ensure you are using the latest version of the Force Regenerate Thumbnails plugin, as updates often include bug fixes and compatibility improvements.
  • Test First: Always test the plugin on a staging site before using it on your live production website.

If you continue to experience issues after trying these steps, searching for your specific error message on the WordPress support forums may yield more targeted advice from the community.

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