When a policykit-1 window opens (say when I want to change a user’s password), it looks ready for a fingerprint and my fingerprint reader flashes, but it will not scan a finger. No matter how many times I run my finger across the reader, it does not read “Not verified” or “OK.” All the other places I use the reader (login screen, sudo prompts, etc.) work fine.
Possibly there is an additional "polkit-gnome-authentication-agent" running. If yes, please remove the file "/etc/xdg/autostart/polikt-gnome-authentication-agent-1.desktop" and restart your system.
http://home.ullrich-online.cc/fingerprint/Forum/topic.php?TopicId=450&Posts=1