How to set Cloud9 URLs to subfolder?

Question

I have installed an instance of cloud9 on a webspace (CentOS 5). Installation was easy and it is running fine.

My hosting provider supports mod_proxy, thus I use a .htaccess to re-route the requests to node:

RewriteRule ^cloud9/(.*) http://localhost:3131/$1 [P]

In theory, this works. I can access it and the sent source is clearly cloud9. However, the page stays white and my JavaScript console shows me this error:

ReferenceError: require is not defined

Looks like they are using require.js, which fails to load:

 <script type="text/javascript" src="/static/require.js"></script>

It is trying to load it from example.com/static/require.js because it doesn't know that it is called through a proxy. What do I have to change so it will use example.com/cloud9/static/require.js?

I have not found anything in the configuration files, but it would be great if there is a solution that does not require me to modify any of cloud9's code.