The problem you have is due to the fact that you cannot open more than
1 port on the same computer.

I had the same situation lately.  The solution I used is quite
simple.  I get the session id from the terminal server and add it to
my port. That assure me I got a unique port for every session.  To get
the remoting session id use the following method:

     [System.Runtime.InteropServices.DllImport("kernel32.dll")]
     static extern bool ProcessIdToSessionId(uint dwProcessId, out
uint pSessionId);

     public long GetRemotingPort()
     {
        uint sessionId = 0;
        System.Diagnostics.Process currentProcess =
System.Diagnostics.Process.GetCurrentProcess();
        bool result = ProcessIdToSessionId((uint)currentProcess.Id,
out sessionId);
        return 65000 + sessionId;
     }

In this example, I removed all error trapping validation.  The base
port i was using is 65000 and if not on  a terminal server, the
ProcessIdToSessionId returns 0.

Nicolas Bouchard