This is probably because your home router doesn’t support hairpin NAT. Basically it doesn’t know how to deal with requests to its own WAN IP.
The typical solution is to either also run a DNS server in your home network to make your domain resolve to your LAN IP instead (requires a bit of effort).
Another way is to just serve a second site in Caddy that doesn’t use a domain with the WAN IP. You could get yourself another duckdns domain that you manually set to your LAN IP (no auto updating) and use that, but over http only because you won’t be able to get a let’s encrypt cert for a domain that doesn’t resolve to a public IP.
This is a startup error, so I figure Caddy wasn’t able to start again. There could be all kinds of reasons for this. Does the user you run the service as have the permissions to open network ports? Do you have another service running using port 80? You’ll need to dig in deeper to find out the cause.