To detect when a user last logged into the domain, we use the Last-Logon-Timestamp property of the user account. However, since your domain is operating in Windows 2000 native mode, this property cannot be used. The Last-Logon-Timestamp property is supported in the Windows Server 2003 mode and higher modes. Probably, you confused the Last-Logon-Timestamp property with the Last Logon property that is supported in the Windows 2000 native mode.
So, since the property that we use to detect when a user last logged into the domain is not supported in the domain mode that your domain is operating, the report cannot be generated correctly. The only thing we can recommend in this case is that you raise your domain functional level to at least Windows 2003 Server mode. Since all your DCs are running Windows Server 2008, this should not be a problem. For information on how to accomplish this task with the help of Adaxes, see Raising Domain or Forest Functional Level.