40D + Canon OC-E3 flash synch cable problem

When this cable is attached to my 40D I cannot access the ETTL flash menu. The message reads "flash power turned off or not compatible". I called Canon tech support and was told it was probably a faulty cable. The hot shoe connection seems to be OK because my 270EX and Sigma macro ring flashes both work when attached directly to the camera. Prior to buying the Canon cable (trying to avoid the $65 price tag) I tried two 3rd party cables (Opteka and Zeikos) and had the same problem. Anyone have any experience or knowledge about this issue?

Thanks.