All "System policy prevents..." and derivative dialogs do not accept correct password

Ubuntu 9.04, all updates installed, no other problems.

Dialogs including "System policy prevents x" or "Authentication is required to x" or any kind of authentication or authorisation dialog that has the option to switch the user, will not accept my known-correct password. However, sudo and su in the terminal work fine, and gksudo and gksu work too, where the background behind the prompt greys and there is no user switch option. Also, screen lock works.

The window immediately shakes when my correct password it put it. When I put in an incorrect password, it hesitates a little and then shakes. Three tries and it closes.

I would like to figure out how to restore functionality to authenticate dialogs, because I cannot mount my Windows drive (it's a dual boot machine), can't change user authorisations, can't log out if I letf a root terminal open in another desk, etc.

I use ext4.

I believe this is some kind of permissions problem with some obscure file.

Expected: The Authenticate dialog accepts the dialog, closes, and whatever administrative task that is supposed to run runs.

Actual: The dialog does not accept the password, does not close, and the task does not start.

Corresponding forum post: http://ubuntuforums.org/showthread.php?p=7602499#post7602499

Thank you.

Thatbum