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Do you see any errors in the error log of your webserver?
You should also look at the view script. It will be in wp-content/cache/ and be named view_xxxxxxxx.php – the first bit of code should contain the username/password.
There’s also a chance you don’t have the correct authentication module installed. If you’re using Apache2, the auth_basic module provides this functionality. You can enable it (as root) with the a2enmod command.