Connection timeout when trying to load non-batched JavaScript files in 5.8.x

Since upgrading Confluence to version 5.8.x, we get connection timeouts after 20s (default connection timeout of HTTP connector in server.xml) when trying to load non-batched JavaScript files.

Steps to reproduce this issue:

1. Use a fresh Confluence 5.8.x instance
2. Try to load any *.js file from the includes folder in your browser. Example: http://localhost:8090/includes/js/moment/moment.js
3. It may be displayed if your browser has already cached the file, but when you force reload the file (shift refresh), the browser keeps loading the file and fails after 20 seconds.

It looks like only JavaScript files are affected by this, images or CSS files are loaded correctly. We were able to reproduce this in any modern browser.

We recognized this when trying to add a user to a group in Confluence. The user picker/search popup doesn't seem to work anymore. It looks like it tries to load the file /includes/js/entity_picker.js, which also results in a connection timeout. We've also added some custom JS files (for analytics) in the includes folder and referenced them in the custom HTML section. This used to work perfectly in Confluence versions < 5.8.x, but doesn't seem to work anymore.

We are using Confluence 5.8.10, but I was also able to reproduce this in a fresh Confluence 5.8.4 instance. Any help would be appreciated.