This error is encountered when attempting to use LDAP queries for authentication on a LAMP server, or a Linux server running Apache. This behavior was encountered on an Ubuntu Linux server.
An attempt to authenticate using an LDAP lookup to an Active Directory domain controller from a web page running on an Apache web server produces the following error in the apache logs, and the user fails to be authenticated.
[ldap_search_ext_s() for user failed][Operations error]
It is not currently understood what the primary cause of this error would be, however the fix points to a misconfiguration in the ldap.conf file.
The solution is to modify the following file /etc/ldap/ldap.conf and uncomment / add the following line to disable referrals.
Once the line has been uncommented / added, save the file and restart apache using the following command.
sudo service apache2 restart
The error will stop being logged, and the user will authenticate correctly allowing access to the website.
none (unable to locate the web page where I found this information. I will update if I run across the site again.)