[nginx] How do I rewrite URLs in a proxy response in NGINX

You may also need the following directive to be set before the first "sub_filter" for backend-servers with data compression:

proxy_set_header Accept-Encoding "";

Otherwise it may not work. For your example it will look like:

location /admin/ {
    proxy_pass http://localhost:8080/;
    proxy_set_header Accept-Encoding "";
    sub_filter "http://your_server/" "http://your_server/admin/";
    sub_filter_once off;
}