Dirsync and Discovery error: "Error connecting to endpoint: The LDAP server is unavailable" (4299721)

Unable to discover domains with On Demand, Directory Sync.

Directory Sync Agent Discovery Logs show the following error :
 

• "Error connecting to endpoint: The LDAP server is unavailable"

Resolution 1:

This general connectivity error indicates a problem establishing a connection with the Active Directory Domain Controller.
 

This may happen if a previous Environment Discovery had resolved the FQDN of a Domain Controller to an IPv6 address. If IPv6 is then disabled on the agent server or an agent is moved from a server with IPv6 enabled to another with IPv6 disabled, the agent may fail to establish a connection.
 

To resolve this, re-enable IPv6 on the network interface and perform another Environment Discovery.
 

Resolution 2:

This issue may also occur if the directory sync agent service account UPN is not valid for the given Active Directory domain that the agent is connecting to. To resolve this, ensure that the UPN is valid for the domain that the agent is connecting to.