importFile() worked fine in CC2017, now in CC2018 it throws an "Illegal Argument" error.

here's a small JavaScript to test. Open any document or create a blank file before running the script

var f = File('c:/temp/Example File.pdf'); // to test, add a path to a valid file in your system

var idoc = app.activeDocument; // have any document open

idoc.importFile(f, false);