if its encrypted, there is no way of opening it (for reading file, which is requiremement for conversion to stl) without encryption keys. And what you have there is Network error: service (server) refuses to accept connection from your computer.
It could be that your computer is not allowed to connect to "Key Exchange" service (which can be local computer service, service running on other computer in your LAN or internet service),which enables you to use encryption keys needed for opening encrypted files.
reasons can be firewall, incorrect software configuration etc etc.
