This is less of a problem and more of a discussion regarding a decision made by XenForo. If you go view the source code of this thread you will see that there's a base tag defined.
<base href="http://xenforo.com/community/" />
and that all references to files are done using relative links.
<link rel="css.php?css=xenforo,form,public&style=2&dir=LTR&d=1395480627" />
This is a smart trick to make sure you can use relative links rather than absolute links. (though I'm not sure why that's desired) The problem is that some of our users are browsing from behind a caching forward proxy so the proxy will try to access the css.php file but then not take the base href into account which causes a 404 in our error log.
Specifically we're having problems with the proxy used on Google campuses. Would anyone happen to know how easy it would be to convert every embedded resource to an absolute link rather than a relative one?