I got one of these doorstops for free (with rebate, which I've not received yet) about 6 weeks ago and I've had this problem as well.
dohc3si has a working solution. I tried signing in with the correct admin user name and password and, of course, got the login page again. At that point I went up into the Address bar in IE and added port 80 to the address, changing the URL from this:
I then hit the enter key. I got the little familiar bubble from the network connection in my tray saying "a cable is unplugged" as the router rebooted (!?!). At least it seemed like that was what it was doing. It eventually finished the reboot sequence but that IE never stopped spinning, so I stopped it and put the basic URL into the address bar:
hit enter, and the main page came up as if I had just logged in correctly.
This isn't really a solution as it causes the router to reboot, but at least it reboots the router without having to pull the power plug.
I wonder if DLink will ever do anything about this.