In case anyone is still struggling with this, I discovered the problem is that in standalone mode the Editor will have started a server instance, and as the standalone game will try to start a server bound to the same port, it will fail.
The solution is to prevent the server from automatically starting (Auto Start Web Server/Web Socket Server in project settings), and to make sure the console command WebControl.StartServer is only triggered in either the editor or your standalone game.
If there is a runtime way to set the web control port, that would make life much easier, but I couldn’t see one!