XSLT stylesheet not being applied by nginx for one upstream server, but working fine for another

Question

I am experimenting a little with Nginx, and using XSLT to transform RSS-feeds from external sites to a unified format.

I first made a working XSLT-stylesheet and applied it to the RSS-feed from bt-chat.com, which worked fine, and gave me exactly what I wanted. I then set up a location for a feed from showrss.karmorra.info, with identical configuration, but for reasons I can't understand, the XSLT-stylesheet just isn't applied. Instead, the XML goes straight through, unchanged.

I can't find any reason for why this should work for one external site and not the other, but I also have trouble understanding how identical configurations could give such different results.

Any tips or pointers anyone can give me would be appreciated. The only difference I can see on the external sites, apart from the XML actually being different, is that one of them is returning a "Last-Modified" header, but I can't for the life of me see how that should affect the XSLT-stylesheet being applied or not.

My Nginx-conf:

user nginx nginx;
worker_processes 1;

events {
    worker_connections 1024;
    use epoll;
}

http {
    include /etc/nginx/mime.types;
    default_type application/octet-stream;

    log_format main
            '$remote_addr - $remote_user [$time_local] '
            '"$request" $status $bytes_sent '
            '"$http_referer" "$http_user_agent" '
            '"$gzip_ratio"';

    access_log /var/log/nginx/access_log main;
    error_log /var/log/nginx/error_log debug;

    index index.html;

    server {
            listen 81;
            server_name theano;
            server_name localhost;

            access_log /var/log/nginx/ibidem.homeip.net-access_log main;
            error_log /var/log/nginx/ibidem.homeip.net-error_log debug;

            root /var/www/ibidem.homeip.net/htdocs;

            location /feed/btchat/ {
                    proxy_pass http://rss.bt-chat.com/?group=2;
                    xslt_stylesheet /etc/nginx/nginx_feed_btchat.xslt
                            showname=$arg_showname;
            }
            location /feed/karmorra/ {
                    proxy_pass http://showrss.karmorra.info/feeds/all.rss;
                    xslt_stylesheet /etc/nginx/nginx_feed_karmorra.xslt
                            showname=$arg_showname;
            }
    }
}