TCP Connection problem

[Las]

Hello,

I'm not sure this is the right group to ask for help but I try

I have one server (actually it is a commercial game) which is listening on a
specific port for TCP/IP connection. I can connect to it either locally or
remotely with one exception. There is one particular computer where the
connection is refused by the server ("No connection could be made because
the target machine actively refused it") only if the client is on the same
machine with the server.

The two program worked fine before, but obviously something had changed on
this computer.

Could you guide me what to look for, what to check?

TIA,

Las