users-admin fails

[dbclinton]

Recently, when I click on the "Users and Groups" GUI (using 10.10), the
dialogue box shows up, but I'm unable to make any changes because I'm
not prompted for authentication (the "add", "delete" etc., buttons are
grayed out). Launching sudo users-admin simply fails, and running
users-admin from terminal without sudo is no different from clicking on
System/Administration/UsersandGroups - except that I get this message:

        (users-admin:1959): GLIB-GObject-critical **:g_object_unref
        assertion 'g_is_object (object)' failed

Anyone have any ideas?
Thanks,
David Clinton