This is so strange. If I fetch the page from http://archive.is I get the same default Nginx page

http://archive.is/AxHFV

the only thing I can think off right now is that the http "Host" header field is not sent. I have several sites on the same server and Nginx is used as a reverse proxy and uses the Host field to redirect traffic to different ports.

You should really consider using multiple server blocks instead of relying on the Host field.

I do. Nginx does the matching using the Host field http://nginx.org/en/docs/http/ngx_http_core_module.html#serv...

Hmm, I could've sworn that server_name doesn't rely on the Host header. My mistake.