Ok in that case review do you have imported LDAP SSL certificate issuer CA in WebSphere trust repository and the WAS certificate issuer CA imported in LDAP SSL trust repository.
If you haven't the CA public certificate interchanged I think that it can't stablish the connection in secure. That is the reason of "socket closed null".
The first step is to interchange CA certificates.
and where are you getting "SECJ7340E: Exception raised trying to connect to LDAP server: NamingException: ldap_server_ hostname:port no; socket closed null."?
look at the ffdd logs (\profiles\\logs\ffdc)
Are you getting some input in LDAP server logs?
regards,