1) Is there a chance there was something else running on machine_name:61720? Can you copy/paste the exact exception?
2) I would guess that there's nothing bound to localhost:61720, whereas there's another process that's bound to machine_name:61720.
3) This is because in the latest activemq versions the URI parameter also controls what network interface the broker binds it's listen port to, so if you do a netstat or lsof you'll see that the broker has only opened 61720 on the loopback interface and will therefore not accept incoming connections on the machine's network card.
4) It could be that your network interface doesn't have multicast enabled. This would work on loopback but not necessarily on your network card, so it's something you'll want to check.
There was no other process using the machine_name:61720
The exception thet was thrown is:
WARN TransportConnection - Failed to add Connection
java.lang.SecurityException: User name or password is invalid.