Cannot update advanced settings when running WP behind a proxy

hi,

I have set up WP on a subdomain blog.example.com and then have some proxy forwarding so the blog appears to be installed at example.com/blog
All the links in WP are pointing to example.com/blog

I’ve enabled the plugin and turned it on but can’t add save the selections because the plugin couldn’t detect the current hostname.

https://www.dropbox.com/s/9q926086lra9rqw/wpsupercache_cannot_detect_host.png?dl=0