You're using the Dualcore Netplay branch, make sure you specify that next time!
Anyway, try disabling all of your firewalls and perhaps port forward (UDP and TCP) a port and force that in Options/Configuration under Netplay Listen Port and try hosting for someone else.
Anyway, try disabling all of your firewalls and perhaps port forward (UDP and TCP) a port and force that in Options/Configuration under Netplay Listen Port and try hosting for someone else.