There is input voltage monitoring, and an overvoltage shutdown if the input voltage goes over spec. It could be possible that if your supply is idling quite high and there is some voltage sensing circuitry on the supply that isn't connected, you could have enough overshoot to trip that protection. It would be hard to see with a multimeter, but you could be going over 13.2V.

Try connecting the voltage sense lines as klondike_bar suggested to get the voltage back to 12V, and see if that fixes the issue.