Normally, Divi Booster loads the following files to add the CSS / JavaScript code it generates to the site:

/wp-content/uploads/wtfdivi/wp_head.css
/wp-content/uploads/wtfdivi/wp_footer.js

If it can't find these files then it won't be able to make changes on the page.

Fixing the WordPress UPLOAD_PATH Setting

The most common reason I've seen for 404 errors on the Divi Booster files is that the WordPress "upload_path" setting is wrong. This setting tell WordPress (and Divi Booster) where the "uploads" folder is, and Divi Booster uses it to determine where to write its files to the server.

Normally, this setting is blank and WordPress uses a default value (i.e. the usual /uploads folder). However, sometimes the uploads_path setting will have been explicitly set on the server. This is fine if upload_path is actually set to the /uploads folder, or if the corresponding "upload_url_path" setting is set to allow WordPress (and Divi Booster) to determine the URLs for the files in a non-standard upload folder. But if the upload_path and upload_url_path do not point to the same place on the server, then Divi Booster will write its files in one place and attempt (and fail) to read them from another place. Hence the 404 Not Found errors.

In particular, I've seen this happen when migrating a site. If the main site has the upload_path explicitly set to the standard /uploads location, and the upload_url_path is left empty, then the main site will work correctly since both values refer to the same default location. But if the site is copied to another location and the values aren't updated, this can cause a problem. Now the upload_path still points to the uploads folder on the main site, while the upload_url_path points to the uploads folder on the new site. As a result Divi Booster will be trying to write to a different location that it tries to read from.

The solution is to correct the upload_path and upload_url_path on the site (the copied site, if you're having the problem after migrating). These values can be edited by going to /wp-admin/options.php on the site (manually type this in, adding your site URL to the start), and scrolling down until you locate fields for these two settings.

Unless you are actually using a non-standard uploads folder location, then simply deleting the contents of these two settings should restore WordPress to the correct default behavior. Here's how it should look: