port forwarding not working

[xoft]

Usually when you try to connect from your own LAN to your own external IP the connection won't work, so this could be the case. The external port checker or one of your friends trying to connect from the outside should be the definitive answer.

I suspect an IPv4/IPv6 issue. From the netstat listing, your Cuberite is listening on IPv6. Do you have IPv4 enabled on the raspi (sorry, can't help you with a command here, it likely depends on your Linux distro)?