Comment 3 for bug 758592

I have this as well, the dialogue is also unhelpful as it is not specified which username is used to verify the password entered with. (Neither the current user's nor the root's password work)

In my case I was opening a website with a video in a tab of akregator for which I didn't have the plugins installed. some gstreamer extra packages were found, but I couldn't install them via this route, since the authentication didn't happen.

I have the same version of kpackagekit installed on Natty.

/Simon