Can't login, SSL error since use of Nginx web server

Hello

So since I setup Nginx instead of Apache a a web server to run a PunBB 1.4.2, we can't login anymore :

Links and config: http://pastebin.com/fd2D5MZ0

When tryng to login at

see pastebin first link

we are redirected to the page that ask use to confirm or cancel the action because a security token could not be confirmed.

Upon clicking on the "Confirm" or "Cancel" we are redirected to

see pastebin second link

Note the use of h ttps and port 80.
As expected, chrome displays a "SSL connection error  ERR_SSL_PROTOCOL_ERROR".

So what's up with that ?
I obviously have something to change in the Nginx config, but so far no luck finding something usefull on the Internet.
I will cross-post this on the Nginx forums, thanks if anyone can help here !

I don't have any SSL certificat installed (and don't want any).
Here is the vHost config :

see pastebin