nginxエラーの意味：/etc/nginx/sites-enabled/mywebsite.confの上流「my.domain.com」にホストが見つかりません。

Debian のアップグレード後、以前に有効であった設定が動作を停止しました。私が見るエラーはsystemctl status nginx.service次のとおりですhost not found in upstream "sync.mydomain.net"。

Nov 20 01:05:40 mydomain.net systemd[1]: Starting A high performance web server and a reverse proxy server...
Nov 20 01:05:45 mydomain.net nginx[442]: nginx: [emerg] host not found in upstream "sync.mydomain.net" in /etc/nginx/sites-enabled/myconfig.conf:9
Nov 20 01:05:46 mydomain.net nginx[442]: nginx: configuration file /etc/nginx/nginx.conf test failed
Nov 20 01:05:46 mydomain.net systemd[1]: nginx.service: Control process exited, code=exited, status=1/FAILURE
Nov 20 01:05:46 mydomain.net systemd[1]: nginx.service: Failed with result 'exit-code'.
Nov 20 01:05:46 mydomain.net systemd[1]: Failed to start A high performance web server and a reverse proxy server.

関連する構成ファイルは次のとおりです。

server {
    include myhttpsconfig.conf;
    server_name sync.mydomain.net;
    ssl_certificate     /var/lib/dehydrated/certs/sync.mydomain.net/fullchain.pem;
    ssl_certificate_key /var/lib/dehydrated/certs/sync.mydomain.net/privkey.pem;

    location / {
        include myhttpssubdomain.conf;
        proxy_pass http://sync.mydomain.net:8383;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

}

失敗の理由は何ですか？