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Troubleshooting Common WebP Express Bulk Convert Errors

40 threads Sep 16, 2025 Plugin → Webp express

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Many WordPress users rely on the WebP Express plugin to efficiently convert their images to the modern WebP format. However, the bulk conversion feature, while powerful, can sometimes run into issues that halt the process. Based on community reports, here are the most common errors and their solutions.

1. "Invalid security nonce" or "security nonce has expired"

This is one of the most frequently reported errors. A nonce is a security token that prevents unauthorized requests. For safety, these tokens have a limited lifespan.

Why it happens: If you leave the settings page open for a long time before starting the bulk conversion, the nonce can expire.

Solution: Simply refresh the WebP Express settings page in your browser (press F5) and click the "Bulk Convert" button again. This generates a new, valid security token.

2. Generic "Error: error", "Bad Request", or 500 Internal Server Error

These vague errors often appear in the bulk conversion popup or the browser's JavaScript console. A status 500 error from admin-ajax.php indicates a server-side problem.

Why it happens: The root cause can vary widely. Common culprits include:

  • Server resource limits (PHP memory, execution time) being exceeded.
  • Conflicts with other plugins, especially caching or optimization tools.
  • Problems with the server configuration (NGINX rules, permissions).
  • A corrupt image file that causes the conversion process to crash.

Solutions:

  • Increase Resource Limits: Try increasing your PHP memory limit (WP_MEMORY_LIMIT), max_execution_time, and max_input_time in your wp-config.php file.
  • Disable Conflicting Plugins: Temporarily disable all other plugins, especially caching plugins, to see if the conflict is resolved. If it works, re-enable them one by one to identify the culprit.
  • Check Server Configuration: If you are on an NGINX server, double-check that the rewrite rules provided in the WebP Express documentation have been implemented correctly.
  • Identify Problematic Files: The process may hang on a specific corrupt image. If the interface shows which file it's stuck on, locate and check that file in your Media Library or file manager. You may need to remove or re-upload it.
  • Convert in Smaller Batches: Instead of converting all images at once, use the scope options to break the job into smaller pieces. Start with only "Uploads", then move on to "Uploads and Themes".

3. "None of the converters in the stack are operational"

This error means WebP Express cannot find a viable method (like cwebp, GD, or Imagick) to convert your images on the server.

Why it happens: The required PHP extensions (GD, Imagick) may not be installed or configured with WebP support on your server.

Solution: Check your server's PHP environment. Contact your web host to confirm that the GD or Imagick extension is installed and compiled with WebP support. You can also run a "Live test" in the WebP Express settings to diagnose converter availability.

4. JSON Parse Errors (Unexpected EOF, Unexpected token)

Errors like "Unexpected EOF" or "Unexpected token o in JSON at position 1" indicate that the plugin is receiving a malformed response from the server instead of valid JSON data.

Why it happens: This is almost always caused by PHP warnings, notices, or errors being printed to the browser, which corrupts the JSON data structure that the JavaScript expects. The sample thread mentioned a zlib output compression error causing this.

Solution: Enable WordPress debugging by adding define('WP_DEBUG', true); and define('WP_DEBUG_LOG', true); to your wp-config.php file. This will log errors to a file (wp-content/debug.log) instead of printing them on screen. Check the log to identify and fix the underlying PHP issue causing the output.

5. Bulk Convert is Unresponsive or "Stuck"

The interface hangs on "Receiving a list of files to convert..." or the progress bar doesn't move, especially on sites with a very large media library (e.g., 90k+ images).

Why it happens: The process of gathering a huge list of files can time out or exceed PHP memory limits before it even begins converting.

Solutions:

  • Use the "Create webp files upon request" (on-demand conversion) mode instead. This generates WebP images as they are requested by visitors, which is much less resource-intensive on the server.
  • As mentioned above, convert in smaller batches by scope (Uploads only) or by image type (JPEGs only, then PNGs).
  • The WebP Express team has acknowledged the need for a background process or WP-CLI command for large-scale conversions, which would be better suited for massive libraries.

General Tips for a Smooth Bulk Conversion

  • Backup First: Always perform a full backup of your site and database before initiating a bulk conversion of thousands of files.
  • Check Permissions: Ensure the plugin has write permissions to the wp-content/uploads-webpc directory (or your chosen cache location).
  • Be Patient: For large media libraries, the process can take a very long time. Let it run undisturbed.

If you continue to experience issues, checking the WordPress debug log and your server's PHP error log is the most effective way to pinpoint the exact cause of a failure.

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