login problem after LDAP server change

Hello

Since we change the machine supporting our LDAP server we get the following
when normal user attempt to log in from a Linux station , below is the transcript
of a su session but the same message appears if we attempt from the KDM login panel.

Feb 26 17:07:27 acme11 su[893]: (pam_unix) expired password for user exam40 (password aged)

We didn't any change on clients

The server software has been re-compiled from the same sources (2.0.27) as on the preceding machine
and the base has been exported and re-imported from a LDIF file ...

Thanks for any help/infos

Frank