Hi, I had same problem and I updated /etc/nginx/sites-enabled/odoo.conf file as below: 

# Odoo server 
upstream odoo {server 127.0.0.1:8069;
}

upstream odoochat {

    server 127.0.0.1:8072;

}

map $http_upgrade $connection_upgrade {

    default upgrade;

    ''      close;

}

server {

    listen 443 ssl;

    ssl_certificate /etc/ssl/certificate.crt; 

    ssl_certificate_key /etc/ssl/private.key;

    proxy_read_timeout 720s;

    proxy_connect_timeout 720s;

    proxy_send_timeout 720s;

    # log

    access_log /var/log/nginx/odoo.access.log;

    error_log /var/log/nginx/odoo.error.log;

    # Redirect requests to odoo backend server

    location / {

        # Add Headers for odoo proxy mode

        proxy_set_header X-Forwarded-Host $host;

        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        proxy_set_header X-Forwarded-Proto $scheme;

        proxy_set_header X-Real-IP $remote_addr;

        proxy_redirect off;

        proxy_pass http://odoo;

        add_header Strict-Transport-Security "max-age=31536000; includeSubDomains";

    }

    location /longpolling {

        proxy_pass http://odoochat;

    }

    location /websocket {

        proxy_pass http://odoochat; 

        proxy_redirect off;

        proxy_http_version 1.1;

        proxy_set_header Upgrade $http_upgrade;

        proxy_set_header Connection $connection_upgrade;

        proxy_set_header X-Forwarded-Host $host;

        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

        proxy_set_header X-Forwarded-Proto $scheme;

        proxy_set_header X-Real-IP $remote_addr;

    }

    # common gzip

    gzip_types text/css text/less text/plain text/xml application/xml application/json application/javascript;

    gzip on;

    client_body_in_file_only clean;

    client_body_buffer_size 32K;

    client_max_body_size 500M;

    sendfile on;

    send_timeout 600s;

    keepalive_timeout 300;

}

server {

    listen 80 default_server;

    server_name _;

    return 301 https://$host$request_uri;

}

proxy_http_version 1.1;

proxy_set_header Host $host;

after save it. You should restart ngingx service :

 
sudo systemctl restart nginx

or 

sudo service nginx stop
sudo service nginx start

If you are still experiencing issues, you can check the Nginx error logs for more information:

tail -f /var/log/nginx/error.log 

I solved this problem with the solution and I hope the solution will help you also.