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Port 80 is usually reserved for HTML access to the router, especially when "Remote Management" is enabled.
I've looked at the Javascript that raises the message and believe that it is incorrect. If D-Link is interested i can probably write a correct version of the code.
I have the same issue. I bypassed it by changing the order of virtual servers. They have some weird looping going on there for the validation. I have public 80 forwarded to private 80 on a server. When I added it to the bottom of the list, I get this error, but when I added it to the top of the list, it works. Ugghh !