I´m facing the same problem here. It´s stranger because the problem only happens in the first 5 or 6 tries, then the document loads. The problem occurs again if the IE is closed (Tried with IE 7,8 and9).
In my case the plugin is the same. I seems that some of the IE or Network configurations is delaying the authentication process (named user).
After you try 4 to 5 times, it all works fine. Make the access work in the first try is the problem. Some machines (like mine!) has no problem. Accessing using QV Desktop open in server works great too!
The error shown in the client Plugin or in the server events log is the same if you cancel the User/Password window or give the wrong password.