FYI I spent 2 hours trying to figure this out a brand new Panel Due with 1m cable from Filastruder for my Duet2Wifi. I kept getting the connecting issue, and was stumped. Glad I found this thread as indeed is a faulty cable; +5, Gnd and Dout were good, but Din was failing when I metered it out. It's really really strange as I don't see any reason for why the cable is bad. The crimps even look good when I take it out of the housing, yet no connectivity (must be an internal break?) I made up a custom wire, and was up and running in no time.
So for those finding this later via Google like me, check the cable!