just try to enter the name (FQDN) of a domain controller and enter username+password that is allowed to read LDAP (standard user will do).
If you instead use the domainname / site thing you may have problems. Not sure why, but I guess it's doing some broadcast to get a valid DC for the domain you entered - and fore some reason this doesn't always work (maybe need the short name - so doing a NETBIOS query for DC???).
I usally enter the DC directly - as mentioned above (without "site").
I already tried connect directly to the DC server, without the use of site and domain name, the same result: Query returned no domain controller records. Verify the server has at least one domain controller and the user has required permissions