upstream prematurely closed connection

[Thierry]

Dear all,

The error  message:
2017/01/05 18:28:49 [error] 32633#32633: *3 upstream prematurely closed connection while reading response header from upstream, client: xxx.xxx.xxx.xxx, server: server_hostname.domain.ltd, request: "POST /SOGo/connect HTTP/2.0", upstream: "https://ip_server:port_number/SOGo/connect", host: "server_hostname.domain.ltd:port_number", referrer: "https://server_hostname.domain.ltd:port_number/SOGo/"
2017/01/05 18:28:49 [error] 32633#32633: *3 upstream prematurely closed connection while reading response header from upstream, client: xxx.xxx.xxx.xxx, server: server_hostname.domain.ltd, request: "GET /SOGo.woa/WebServerResources/busy.gif HTTP/2.0", upstream: "https://ip_server:port_number/SOGo.woa/WebServerResources/busy.gif", host: "server_hostname.domain.ltd:port_number", referrer: "https://server_hostname.domain.ltd:port_number/SOGo/"

I  have  a front-end reverse proxy using Nginx for my web  server and 
for the web interface of my email server using Sogo.

From  my  firewall:  
-  If I NAT the "Sogo" port to my reverse-proxy I do 
have these error and a 502 Bad Gateway  message.
-  If I bypass my reverse-proxy and NAT  directly to my email server, 
this is working.

On my reverse-proxy server, I am using Nginx version 1.9.10 (debian 8)
On my email server, I am using Nginx version 1.6.2 (debian 8)

If  you  need  more information, please  ask ... I  do not understand 
this error myself.

ps: it is  the second time  I am  sending this email (?)
  

-- 
Cordialement,
 Thierry                          e-mail : lenaigst at maelenn.org  
 PGP Key: 0xB7E3B9CD