This is a curious one I’ve seen a few times in the past, and once recently, if I recall.
For some reason, Caddy is trying to use its own host server ([::1]:53) for DNS resolution.
I’m guessing you don’t run a DNS resolver on your Caddy host server - hence, when it tries to connect to do a DNS lookup, it the connection is rejected.
I don’t know if it was definitively resolved as to the cause of this issue. In the most recent case, we talked a bit about how Go programs do DNS resolution, specifically in the net/http module. The person with the issue eventually reinstalled Caddy and the systemd unit file they were using, but that was with v1.