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Is your router using UPnP or port forwarding. I suspect the same port use is the issue. On the mac, try changing the listening port to another higher one between 49152 - 65535. Try port 64049. ie: go to LW's prefs, Advanced > FIrewalls & change listening port. Then reboot the mac if you've been using LW on the pc.

I don't know about UPnP on mac, but I do know if I've used a program using one port & try to use the same port for another app then it seems to refuse the connection. So I thus forward a port for each program & each LW. That seems the only way.