I am using QV 10 SR3, VS 2010. I use the call
QlikOcx.OpenDocument(DocPath, myusername, mypassword)
I find that if either the name or password are not correct ... then it asks for both.
Do you have case sensitive name/pswd?
Try using a notepad document that has the name/password and do copy/paste from the notepad to the dialog to make sure these are the correct name/pswd. Then copy/paste these values into your OpenDocument call. I've had some "fat finger" problems in the past that were resolved using a notepad ...