Maybe the Proxy Adapter is already connected on the same port?

Please note that if the Proxy Adapter (on behalf of the Lightstreamer Server) didn't detect any disconnection and so considering itself connected, it does not open the ports for listening.
Hence, it has no knowledge at all of external connection attempts of the new instance. The connection attempt by the Remote Adapter is directly refused by the operating system.

The comunication between Remote Adapters and ProxyAdapter are implemented over TCP sockets created directly by the Remote Adapter. So, if you detect such a situation (that is, a Remote Adapter that fails to connect on a port), you should verify whether the Server is really connected on that ports.
So, if you see an established connection on a port, you can identify the related remote host and remote port.
Then, on the remote host, with netstat or a similar tool, you can see if the system can detect the PID of the process holding the connection.

Regards,
Giuseppe