502 Gateway Error - when accessing postgresql database

[Nanoq]

Hi I'm building a website with nginx, uwsgi and a postgresql database. When
I want to access the database with 

curl -i -X POST -H 'Content-Type: application/json' -d '{"username": "test",
"password": "test", "email":"test"}' xxx.xxx.xxx

I get the following error: HTTP/1.1 502 Bad Gateway

The nginx/access.log file tells me: "POST /main/newuser HTTP/1.1" 502 173
"-" "curl/7.35.0"
and the nginx/error.log file says: upstream prematurely closed connection
while reading response header from upstream
request: "POST /main/newuser HTTP/1.1", upstream:
"uwsgi://unix:/var/www/mysite2/mysite2.sock:"

Everything else, beside accessing the database seems to work. When I run the
my webserver on the localhost the connection with the database works.

Here my configuration files:

sites-enabled/project

upstream db {
    server xxx.xxx.xxx:5432;

}

server {
    listen 80;
    server_name xxx.xxx.xxx;

    location / {
        proxy_pass     http://db;
        proxy_redirect off;
        include uwsgi_params;
        uwsgi_pass unix:/var/www/mysite2/mysite2.sock;

}
}


the uwsgi.ini file;
[uwsgi]
module = wsgi

master = true
processes = 5

socket = mysite2.sock
chmod-socket = 660
vacuum = true

die-on-term = true

python-path = /var/www/mysite2

Thanks in advance for your help!

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,270976,270976#msg-270976