Whenever an external CSS/JS file has to be altered due to any of the settings applied in the plugin (this includes: Minify/Combine CSS/JS, altering @font-face for CSS, altering Google Fonts URL, inline large CSS/JS files), a storage directory for the minified & concatenated files are needed so that the optimized files are retrieved much faster when the pages load, instead of doing the changes “on the fly” and use more resources. The default path is /wp-content/cache/asset-cleanup/
On certain hosting platforms such as Pantheon, the number of writable directories is limited and in this scenario, you have to change the ‘cache’ directory to ‘uploads’.
To apply the change, you have to add the following code to wp-config.php file within the root of your WordPress installation, where other constants are defined, above the line /* That’s all, stop editing! Happy blogging. */
define('WPACU_CACHE_DIR', '/uploads/asset-cleanup/');