Note: When you restart nginx after the configuration change, it might fail if you have another configuration with server block as default for port 80 (in most cases for the nginx welcome page). You will have to disable this config. Most probable places for it to exist are /etc/nginx/conf.d/default.conf and /etc/nginx/conf.d/default . In the latest nginx installs, the file is in /etc/nginx/sites-available/default
Check the frappe-bench/config/nginx.conf for port of site yy.yyy.com it might be something other than 80 because 80 is successfully served by nginx default and nginx didn’t fail to start.