If you see this error when trying to run a web app in Visual Studio – that means something is currently running on that port.
To find out what app is using that port…
Go to Network tab, Listening Ports
Find 443 (or whatever port VS is complaining about)
And see what app is using it – and kill it.
In my case Skype was using 443.
Signing out of Skype solved the problem.