I assume you saw this from your link:
ldap_start_tls: Server is Unavailable
Error Description - The MX uses TLS to secure the LDAP connection to the domain controller. This error indicates the MX received an Error initializing TLS response from the domain controller when attempting to establish TLS.
Error Solution: To resolve issues with TLS, please verify the following:
- The domain controller has a valid certificate installed.
- The domain controller supports STARTTLS. Since the MX does not support LDAP over SSL, it uses STARTTLS instead.