Nginx + Reverseproxy

Hello,

I use nginx as a reverserproxy in front of a IIS Website.

Some page returns me a error "502 Bad Gateway". In the error logs, I can see :
2013/03/27 12:31:34 [error] 310#0: *279 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 10.142.1.72, server: mysite.com, request: "GET /MosStyleMgr/aspx/arboStyle.aspx?style=ui_eex_1&refDossierStyle=../../MosData/MosStyleMgr/styles/ui_eex_1/ HTTP/1.1", upstream: "http://W.X.Y.Z:80/MosStyleMgr/aspx/arboStyle.aspx?style=ui_eex_1&refDossierStyle=../../MosData/MosStyleMgr/styles/ui_eex_1/", host: "www2.elearning.powernextsa.llg", referrer: "http://mysite.com/MosStyleMgr/aspx/menu.aspx?ncnx=25949"

In my vhosts, I put only the directive : 'proxy_set_header X-Forwarded-Host $host;' (necesary for the licence of the ISS site)

Do you have an idea about the origin of my problem?

Regards