I have nothing on the Comcast ISP in my office besides the gateway they provided me plugged into the cable and I logged into that gateway and there is nothing that would be blocking it. I had the same problems with an ATT network down at convention center in South Florida. Here is another thing I have noticed. If I disable the local LAN adapter on the security server, flush the DNS, and re enable that local LAN adapter, it will work (usually) connecting to it. If I disconnect from the desktop and try to reconnect, it will not work, so it works for the inital connection.
Of all the things I have read about this similar issue, people have said it was either a DNS or routing issue so I have no idea.
Next question would be for the DNS entry, do I delete the blast HTML entry or my original External URL entry?