the url in your config has to match the fqdn of your site. Also, I don't believe the https is necessary in that config as you are not using https in your nginx config.

{
"url": "http://the.exact.url",
"secret": "xxxxxxxx",
"database": "mongo",
"port": 4567,
"mongo": {
"host": "127.0.0.1",
"port": "27017",
"username": "nodebb",
"password": "xxxxxxxxxx",
"database": "nodebb"
},