I did start to tell a long sorry tale about how I figured this one out, as far as sniffing the bus with a scope etc, but to save you from the anecdote, it turns out there is a design flaw relating to the PCB lands and the Micro B receptacle. For some cables, the D- line was getting shorted to ground, but only when a micro B cable was inserted, and only with
some cables, including the one provided, which rather unhelpfully in this case you are warned to use and no other. When it was working, I happened to be using a different cable I had lying around that
did work as it had slightly different physical characteristics..
To fix, lift the USB Micro B receptacle, pop in a carefully crafted piece of Kapton tape partially covering about a third of the five pin lands (see second pic below). Resolder the USB Micro B and check pin 2 and pin 5 (or the shield) are not shorting when an offending USB cable is inserted.