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Troubleshooting 404 Errors for Autoptimize Cache Files

45 threads Sep 16, 2025 Plugin → Autoptimize

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If you've ever seen a 404 error for a file in your wp-content/cache/autoptimize/ directory, you're not alone. This is a common issue reported by users of the Autoptimize plugin. This guide will explain why this happens and walk you through the most effective solutions to get your site running smoothly again.

Why Do These 404 Errors Happen?

At its core, a 404 error for an Autoptimized file means that a browser or search engine bot (like GoogleBot) is trying to load a CSS or JavaScript file that no longer exists in the cache. The threads reveal several common causes for this:

  • Stubborn Page Caching: The most frequent culprit is another page caching layer (e.g., from your hosting provider, a plugin like WP Fastest Cache, or a server-level cache like Varnish). This cache stores old HTML that contains links to outdated Autoptimize cache files. When the Autoptimize cache is cleared, those linked files are deleted, but the old HTML in the page cache is still served, pointing to now-missing resources.
  • File Permission Issues: Autoptimize may be unable to write new cache files due to incorrect filesystem permissions on the wp-content/cache/autoptimize/ directory, preventing it from creating the files your site needs.
  • Aggregation Settings: The 'Enable 404 fallbacks' feature, which redirects requests for missing files to a fallback, only works when CSS/JS aggregation is enabled. If aggregation is off, the fallback files may not be generated.
  • Migration or Server Changes: Moving to a new host or server can change file paths or permissions, breaking Autoptimize's ability to function correctly.

How to Fix 404 Errors for Autoptimize Files

Follow these steps to diagnose and resolve the issue.

Step 1: Clear All Caches

Your first step should always be to clear every cache on your site. This is the solution in many of the sample threads.

  1. Clear the Autoptimize cache from its settings panel.
  2. Clear any other caching plugin's cache (e.g., W3 Total Cache, WP Fastest Cache).
  3. If you are on managed hosting (like WP Engine, Kinsta, or Cloudways), use your host's dashboard to clear their server-level or CDN cache. If you're unsure, contact your host's support and ask them to purge the full page cache.

Step 2: Verify File Permissions

If Autoptimize shows a warning that it "cannot write to the cache directory,&quot you must fix the file permissions. The PHP process needs write access to the wp-content/cache/autoptimize/ folder.

  • Using an FTP client or file manager, navigate to the wp-content/cache/ directory.
  • Ensure the autoptimize folder has permissions (CHMOD) set to 755 or 775. You may need to temporarily set it to 777 to test, but 755 is more secure.
  • If changing permissions does not work, contact your hosting provider's support. They can ensure the correct system user has ownership and write access to the directory.

Step 3: Investigate 404 Fallbacks

The 'Enable 404 fallbacks' option in Autoptimize can help mitigate errors by serving a fallback file if the original is missing. However, this requires two things:

  1. The option must be enabled in Autoptimize settings.
  2. CSS and JS aggregation must also be enabled for the fallback files to be generated.

If you are using Nginx, you must also add the required server configuration rules for the fallbacks to work correctly, as outlined in the plugin's FAQ.

Step 4: Check for Plugin Conflicts

Conflicts with other plugins, especially other caching or minification plugins, can cause these issues. As seen in the threads, conflicts with WP Fastest Cache (WPFC) are common.

  • Disable Minification in Other Plugins: If you use another caching plugin, ensure its CSS and JS minification features are turned off. Let Autoptimize handle this optimization to prevent conflicts.
  • Test with a Default Theme: Temporarily switch to a default WordPress theme like Twenty Twenty-Four to rule out a theme-specific issue.
  • Disable Other Plugins: Deactivate all other plugins except Autoptimize. If the problem is resolved, reactivate your plugins one by one to identify the one causing the conflict.

Step 5: Review Caching Configuration for Logged-In Users

If the problem only occurs for logged-in users (admins, editors), the issue is often related to how page caching handles authenticated traffic.

  • In Autoptimize settings, ensure the option "Also optimize for logged in editors/ administrators" is unchecked. Optimizing the admin area can cause problems.
  • Configure your page cache to not cache pages for logged-in users at all. This is often a setting in your caching plugin or hosting dashboard.

When to Seek Further Help

If you have worked through these steps and the 404 errors persist, the problem may be more complex. Your next step should be to contact your web hosting provider's support. They can help you investigate server-level issues, permissions, and their specific caching configuration. When asking for help, provide them with the exact error messages and the steps you have already taken.

By methodically working through these common causes, you can eliminate those frustrating 404 errors and ensure your Autoptimize plugin works as intended to speed up your site.

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