Check the network connectivity between the EEM Server and the configured LDAP server. Also make sure the specified port is correct and open through the firewall. Your LDAP admin should be able to verify the correct port that would be used in your environment.
If the connectivity is good, make sure the password specified for the UserDN is correct. A common root cause for this issue is a password change for this user. If the password changes for the UserDN, it must be updated manually in the EEM/LDAP configuration. A less common root cause related to the user would be that the user does not have permission to query ldap. This would need to be checked by your LDAP admin.