Starting "SQL*Loader" from within Toad, one gets:
SQL*Loader: Release 12.1.0.2.0 - Production on Mon Dec 4 08:06:22 2017
Copyright (c) 1982, 2014, Oracle and/or its affiliates. All rights reserved.
SQL*Loader-704: Internal error: ulconnect: OCIServerAttach [0]
ORA-12154: TNS:could not resolve the connect identifier specified
But, when testing manually:
C:\Users\*the_windows_user*>sqlldr.exe app_user/app_pass@connect_name control=z:\testing_folder\the_ctl.ctl
The error is not there.