Could also be a stale DNS cache entry on one device or the router. If you ping your duckdns fqdn from the device that can’t connect while on your home network, does it resolve to the correct public IP?

I still think a firewall/nat issue is more likely tho.

What is your router make and model? You need to enable hairpin NAT.

Process Explorer (the sysinternals tool) will show you which process has the handle open to the file.