The port number that RMS express refers to if probably the WInmor Virtual TNC command port and has nothing to do with Flex. See WInmor winlink setup>Winmor TNC setup.. Mine defaults to 8500. I would suggest closing and restarting the PC and RMS espress.. If that does not resolve the issue you can choose a different port number ( 8550 ?? ) and restart RMS express.
RMS Express does uses CAT/DAX ports to connect to SSDR not tcp/ip ports.