Abaixo corrigimos o problema:
location ~ ^/300(0|1|2) {
rewrite ^/300(0|1|2)(/?)(.*) /$3 break;
proxy_pass http://localhost:300$1;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
}