ADsOpenObject() has failed with LDAP error 0x8007200f (- 2147016689) The directory service is unavailable

Question

Greetings! When I call ADsOpenObject(LdapPath, userName, password, dwReserved, IID_IADs, (void**)&pObject); it is failed with "0x8007200f", whereas the LdapPath is like LDAP://<Server Address>/DC=AD_domain,DC=com. Please refer to the code snippet below, thanks!

IADs *pObject = NULL;
HRESULT hr = CoInitialize(NULL);
if ( FAILED(hr) )
{
    return hr;
}

DWORD dwReserved = ADS_SECURE_AUTHENTICATION; //ADS_SECURE_AUTHENTICATION = 1

hr = ADsOpenObject(LdapPath, //LdapPath=LDAP://<Server Address>/DC=AD_domain,DC=com
	userName,
	password,
	dwReserved,
	IID_IADs,
	(void**)&pObject); //Here it is failed with 0x8007200f.

if (FAILED(hr))	
{
	if(LdapPath!=NULL)
		delete[] LdapPath;
	CoUninitialize();
	return hr;
}
else
{
	. . .
}