If you are checking your port forwarding, make sure you are doing BOTH TCP AND UDP. The last few times I had issues, it was from UDP traffic on IP's 124.150.152.95, 124.150.152.96, and 124.150.152.99--an extra lair of local security was blocking the IP's, added them to whitelist.

Check the logs of your security software and if you see it blocking IP's in the 124.150.152.xxx range, it may well be your problem. SE leases this IP block...actually a much bigger one, but these are the only ones I've run into so far. Add them to the exception list for that software and it may fix it for you.