Ultimately, the problem turned out to be something different: my proxy configuration inserted the X-Frame-Option DENY into the response headers. Removing that line from my nginx configuration solved the issue.

While I was at it, I also changed the proxy configuration to use HTTP 1.1 instead of 1.0:

    proxy_http_version      1.1;

Hint for anybody who may try to track down similar problems: the Firefox plugin Live HTTP Headers was invaluable in solving this mystery.

I would like to use it behind a reverse proxy (nginx). This seems to work partially, but AJAX calls seem to time out instead of succeed.

Is there any information about how to make such a setup work?

Here is the relevant part of my nginx.conf:

  location /accounting {
    proxy_pass https://<myinternalserver>:443/accounting;
    proxy_set_header        X-Real-IP       $remote_addr;
    proxy_set_header        Host            $host;
    proxy_redirect          off;
    proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_connect_timeout 90;
    proxy_send_timeout 90;
    proxy_read_timeout 90;
    client_max_body_size 8m;
    client_body_buffer_size 128k;
    proxy_buffering off;
    add_header X-Frame-Options DENY;
  }