actually this seems to be a problem with authldap module. i've also experienced same problem. it looks like for whatever reason the ldap connection has terminated from ldap side, but authldap module cannot determine from the connection pointer whether the connection is open or not so it uses the connection and fails. in my particular case the problem was the ldap connection was idle for more time than idle timeout of ldap so ldap terminated it. i've not yet found a satisfactory soln to the problem. though mr. sam has suggested on the list that we should increase ldap idle timeout to be large i.e 1 or 2 hours.