@mholt, do you have a suggested thing that Caddy users should do in this situation? Maybe webroot?
@sritub, you’re right that Caddy is using port 80. You have at least three options (in order from best to worse, in my opinion):
(1) Just use the Caddy-obtained certificate for ZNC. (The certificate isn’t specific to a port number!) You could either reconfigure ZNC to refer to the existing Caddy certificate or copy (or, better, symlink) the Caddy certificate to the location that ZNC expects.
(2) Use --webroot and tell Certbot a location where it can place static files in order for Caddy to serve them.
(3) Shut down Caddy temporarily and then use --standalone (but in this case, you’ll need to provide --pre-hook and --post-hook options to shut down and restart Caddy if you want Certbot to be able to renew the certificate automatically, since Caddy would need to be started and stopped every time for the certificate renewal process).