I will do so.

Two questions:
1/ In my config file /etc/nginx/nginx.conf, in section http, I already have the logging entries defined :
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;

I just tried to put debug as for example : "access_log /var/log/nginx/access.log debug;"

But:
Restarting nginx: nginx: [emerg] unknown log format "debug" in /etc/nginx/nginx.conf:36
nginx: configuration file /etc/nginx/nginx.conf test failed

I also tried to adapt to add the server section that I don't have in my own config file, as your link mention (http://nginx.org/en/docs/debugging_log.html#memory).

I did the same in one mentionned file : default_ssl
#access_log off;
#error_log /usr/share/nginx/www/jeedom/log/nginx.error;
access_log memory:32m debug;
error_log memory:32m debug;


Restarting nginx: nginx: [emerg] unknown log format "debug" in /etc/nginx/sites-enabled/default_ssl:8
nginx: configuration file /etc/nginx/nginx.conf test failed

I a doing something wrong ?

2/ In error log, even if I can't set a debug level, there is already something strange I would like to change:

2015/05/02 13:25:05 [error] 2144#0: *4926 upstream prematurely closed connection while reading response header from upstream, client: XX.XX.XXX.XXX, server: , request: "GET /socket.io/?EIO=3&transport=polling&t=1430565786187-49&sid=IIJ1gX_E4Ny_ojN8AACB HTTP/1.1", upstream: "http://127.0.0.1:8070/socket.io/?EIO=3&transport=polling&t=1430565786187-49&sid=IIJ1gX_E4Ny_ojN8AACB", host: "hostname.dtdns.net:9876", referrer: "https://hostname.dtdns.net:9876/jeedom/index.php?v=m&"

How host and referrer can be filled with a dynamic dns name ? How nginx is aware of this information ? I am requesting with an IP address, so no chance this information come from the sender.

Could this configuration be erroneous ? (/etc/nginx/sites-enabled/default_ssl)

location /socket.io/ {
proxy_pass http://127.0.0.1:8070/socket.io/;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";
proxy_set_header Host $host;
proxy_redirect off;
proxy_read_timeout 6000;
}


Thx.
GuiPoM