Is the IP range of the Terminal Server routable to the Internet? A lot of private networks use addresses like 192.168.0.0 which are non-routable outside the network.
Another guess would be a firewall in place. Are you certain you're able to get through the firewall to the server?
Event Type: Error Event Source: TermDD Event ID: 50 Description: The RDP protocol component "DATA ENCRYPTION" detected an error in the protocol stream and has disconnected the client.