su stopped working (after upgrade to squeeze)

[Hanspeter Kunz <hkunz@ifi.uzh.ch>]

Hi list,

I have a strange problem on one of my debian squeeze boxes. when I try
to become root (or any other local user), su fails with

	su: Authentication failure

and no, the password is correct :)
In the auth.log I see

	unix_chkpwd[4941]: could not obtain user info (root)

If I try to su to an LDAP-user, su fails with

	setgid: Operation not permitted

and I see again the "unix_chkpwd" message in the auth.log.

It seems there might be something wrong with the suid mechanism, but:
* su is running as root
* sudo works without problems, i.e. I can do all of the above with 
	sudo su [username]
  without problems.

My best guess is, that the problem is with my pam-configuration, but I
have a lenny box with exactly the same pam-config that runs without
problems.

So any hints where to start looking are greatly appreciated.

cheers,
Hp