nginx returns 503 over https

Question

this is my stack:

• a single tomcat back-end located at 10.3.1.125 with 2 web apps (website1 and webservice1)
• an nginx server front end located at 10.3.1.19 with a public ip that servers both apps over https with client authentication enabled
• a client app installed on windows mobile 6 devices connected through a GSM/Cellullar Network that consumes webservice1 with personal and ca certificates installed

most of the time the client app works ok, but recently the nginx server started sending 503 errors to some of the clients, these clients are unable to talk to webservice1 but they can browse website1 using ie mobile, what can be causing this situation ?

this is my nginx conf:

server {
    listen       443;
    server_name  xxx.xxx.x.xx;

    ssl                  on;
    ssl_certificate      ssl/qmovil.crt;
    ssl_certificate_key  ssl/qmovil.key;
    ssl_client_certificate ssl/dlloweb-ca.crt;
    ssl_verify_client on;

    ssl_session_timeout  5m;

    ssl_protocols  SSLv2 SSLv3 TLSv1;
    ssl_ciphers  ALL:!ADH:!EXPORT56:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP;
    ssl_prefer_server_ciphers   on;

    location / {
        #root   html;
        #index  index.html index.htm;
        return 503;
    }

    location /QMLocal {
        proxy_pass          http://10.3.1.125:9091/QMLocal;
        proxy_set_header    X-Real-IP $remote_addr;
        proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header    Host $http_host;
        proxy_read_timeout 120;
        #proxy_redirect http://xxx.xxx.x.xx/QMLocal https://xxx.xxx.x.xx/QMLocal;
    }

    location /QMovil {
        proxy_pass          http://10.3.1.125:9091/QMovil;
        proxy_set_header    X-Real-IP $remote_addr;
        proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header    Host $http_host;
        proxy_read_timeout 120;
        proxy_redirect http://xxx.xxx.x.xx/QMovil   https://xxx.xxx.x.xx/QMovil;
    }
}

where xxx.xxx.xx.x is the public ip