When installed with the debian repo, Caddy is set up to run as a systemd service. This means that it runs an instance of Caddy for you automatically. If you try to run a second one, they will conflict.
Instead of using sudo caddy start, you should use the sudo service caddy <action> commands to control Caddy. Your configuration file is found in /etc/caddy/Caddyfile. To reload your config after making changes, run sudo service caddy reload. To see the logs, run journalctl -u caddy --no-pager | less.