Nginx – Reverse proxy domain subdirectory to another host serving the same domain name

I have a Debian 6.0 webserver correctly running with the following setup:

Nginx as front-server listening on the WAN facing interface port 80
    Serving the domain HTTP mydomainame dot org (via reverse proxy to Apache)
    Directly serving and caching static files

Apache as backend-server listening on localhost port 8080 and WAN facing interface port 443
    Serving the domain HTTP mydomainame dot org (behind Nginx reverse proxy)
    Directly serving SSL for HTTPS mydomainame dot org (port 443)

I have to deal with another server that I don't manage which is supposed to serve only HTTPS mydomainame dot org/subdir.

So this external server should be listening on (WAN) port 443 and serving the subdirectory /subdir under the same domain name (mydomainame dot org).

Note that the DNS records for mydomainame dot org point to my Debian server.

The very first thing I tried was to get my Nginx setup so that it would reverse proxy any /subdir request to the external server IP:

  location ^~ /subdir {
    proxy_pass https://000.000.00.000;
    include /etc/nginx/proxy_subdir.conf;
    error_log /var/log/nginx/mydomainame_subdir.errors.log;
    access_log /var/log/nginx/mydomainame_subdir.access.log;
    expires off;
  }

Then set the headers like so:

proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $remote_addr;
proxy_set_header X-Forwarded-Host $http_host;
proxy_set_header Host "mydomainame dot org";

What happens in this case is that the external server bounces back the request with a 301 and redirects to HTTPS mydomainame dot org/subdir which the DNS records basically resolve back to my Debian Apache server listening on port 443.

I think this happens because the application running on the external server (Magento) rewrites the base_url to HTTPS mydomainame dot org/subdir, but I am not fully sure of this fact.

So a browser ends up hitting my server but Apache is not configured to serve /subdir so of course I get a 404.

Should I setup my Apache listening on port 443 to yet again reverse proxy back to the external server?

It sounds like a good way to create a redirect loop so I am a bit sceptic.

I think the problem is actually on the external server but I am not exactly sure of what to suggest to the maintainers of that host.

They too use Nginx, but serving SSL directly on port 443.

Thanks for any suggestion on this mess!