Ok, so I know this post is incredibly old but I am having the exact same problem with no solution.
I am able to login locally to the DVR using admin and a USER group account. I log out of all accounts locally to the DVR. From my PC with the client software, I get the "login failed" for both accounts. When I try using http://: from my PC I get the login page. Trying to login with either account from here doesn't work. It just looks like the page refreshes and the fields are cleared. I am able to access the login page remotely using http://: but I have the same problem there.
I have a NetGear router so I don't have the WAN option listed here in this thread. I don't have a web site hosted on port 80 either. Does anyone know anything else that I could check?