Thanks for the advice. I have not fixed it yet, but I think I know what the problem is.
I have the 100ohm resistor as RLED, but there is nothing in D4. So, the backlight cathode is connected to Vo, but nothing pulls Vo toward Vss, so they happily float together close to Vdd.
I may have misunderstood something, but in light of the schematic, the assembly instructions may be misleading.
The 1.4u instructions say: (Oct 2012 PCB Update: solder RLED, a 100 ohm resistor located at the top of the PCB.)
The rest of the cell on the page is greyed out, which makes the impression that nothing should go into D4.
Looking at the component list, it does not say anything about RLED, but lists a 100ohm resistor as D4.
Also, there was no leftover on the bench when I finished.
I guess the easiest fix is to put a 1N4148 into D4. I will try that tomorrow (it is past midnight in AU).