Here is the problem. Each of the LDAP variables has what is called a heuristic value assigned. Most variables have a heuristic value of 18 which means Anonymous users can access them. The two variables you mention have a value of 20 - meaning they cannot be read by anonymous users.
The solution is to change your call to collect the information as a non anonymous user.
My code looks something like this 😃im objnamespace
Set objnamespace = GetObject("LDAP:")
Set objIADs = objnamespace.opendsobject(bstrMailboxPath,<user>,<password>,1)
Hope this steers you in the right direction.