nss_ldap: could not search LDAP server - Server is unavailable

You can see this message when the LDAP server is down or bad performance.
if the LDAP server is down, you can solve this issue only the LDAP restart.
but, if it caused by bad performance, you should check about connection counts of LDAP.

Local Account(In Base Authentication) is describing their account from LDAP server when nss_swich of the server is configured for LDAP system and the account is trying to use any processes or tasks, jobs.

Sometimes it caused by increasing sessions and decreasing performance of LDAP system.

You can ignore accounts to access to LDAP when the accounts is working their processes or tasks, jobs.

You can reduce a lot of session from local account adding as below the option
URL : http://linux.die.net/man/5/nss_ldap

nss_initgroups_ignoreusers This option directs thenss_ldapimplementation of initgroups(3)
to return NSS_STATUS_NOTFOUND if called with a listed users as its argument.

$ more /etc/ldap.conf
... Add below a line
nss_initgroups_ignoreusers root,nagios,nrpe,www,rancid,oracle,mysql,ntp,postfix,daemon,named