gdm-simple-greeter or pam?

[frist]

I have Fedora 13 installed at home on a PC with several users.

In the last couple of weeks, some users can no longer login at the
gdm-simple-greeter, which is the graphical interface for letting people
click on their name and get a password prompt.

What is supposed to happen is that you click on a user's name, the greeter
window changes to a login prompt, and you type in your password.

What DOES happen is that you click on the user's name, and the
greeter changes its appearance for a moment, and then returns to
the list of users, without giving a password prompt.

This problem even applies to Other. If you click on Other, you should
get a prompt that would let you type in your userid, but instead, get kicked back to the
simple-greeter.

This means that there are some users who cannot login at the console.
The bizarre thing is that some other users are not affected.

For example, if user 'sally' clicks on her userid at the console on computer1, we see the following
messages in /var/log/secure

Mar 27 10:35:03 computer1 pam: gdm-password[8973]:
pam_unix(gdm-password:auth):
conversation failed
Mar 27 10:35:03 computer1 pam: gdm-password[8973]:
pam_unix(gdm-password:auth):
auth could not identify password for [sally]
Mar 27 10:35:03 computer1 pam: gdm-password[8973]: gkr-pam: no password is
available for user


There is in fact a password for this user, because the user can login
at the command line by ssh. The entries in /etc/passwd and /etc/shadow look comparable for all users.

This problem started with a single user, and then over time, other
users became affected.

Notably, all users CAN login at on our Linux Terminal Server Project
X-terminal. LTSP runs ldm, rather than gdm, and just has a simple
box for typing in your userid. It does not use gdm-simple-greeter.

Is it possible that PAM keeps some sort of list of data for different
users, and
that this list has become corrupted? I don't really know anything about PAM.

(Note: I have attempted to turn off the simple-greeter, but there are
documented
bugs in the simple-greeter that make this option impossible in Fedora 13).

Finally, I have done a fresh install of Fedora 14 on a laptop, completely wiping the hard drive and installing from scratch. This problem also occurred on the Fedora 14 laptop after a few logins.