We use Active Directory authentication (not using LDAP). There are two errors in sequence; see below.
Have you tried connecting to the LDAPS port for global catalogs (3269) using another method such as ldp.exe?
That may give you some more information about why the connection is failing. With a connection reset error, it is possible the connection is being blocked by or timing-out on a firewall (either internal or external to the domain controller in question). The Active Directory connection does use LDAP to query the AD structure for users/groups, and connections will be made on 3269, 3268, 636, and 389, depending on whether LDAPS is configured with a proper certificate and the binding options set within group policy.