[LB]Bad root document set via proxy_pass

Hello everybody,

I am using several instances of Nginx (1.4.1). One as a LoadBalancer and the others to host rails applications (using module passenger).

Depending of the url, I forward to the proper backend:
location /<myApp> {
proxy_pass http://<myApp>_cluster;
}

But via the proxy, the web page of my application is not correctly render. The path to load images files is incorrect and point to the default root document location.

Direct access to my application is fine so I guess I miss something in the load balancer configuration but what ?
Before that when I didn't need to precise the url in the location directive, it worked fine via the load balancer as well.

Cheers,
Mat.