I don't have the schematics for those boards (unless they are the same as the ones I have for my WPC/Addams Family). However, I'm pretty familiar with how the Williams electronics guys design things.
In my schematics, I can see that the Williams ASIC on the CPU board has a zero cross input on pin 71.
If you're not familiar with zero cross detection, this is a method of looking at an AC signal (i.e., a sine wave) and detecting when that signal crosses the zero point. If you look at a sine wave, it starts at the zero point, rises to + full scale, falls to - full scale (through the zero point) and back up to + full scale (again, through the zero point).
This zero point is the ideal time to switch on any electronics that uses that sine wave - because it will draw no current at that point.
The zero cross input actually comes from the power driver board. That board uses a couple of differential comparators connected to an AC input from the power supply (across a bridge rectifier). The output from these comparators generates a pulse when the AC signal crosses the zero point.
The CPU board expects to see this signal regularly (either 100 or 120 times a second depending on where you are in the world). If this signal is absent or wrong, I would expect to see the error you reported. A further clue is that you said this happened when you shorted out a controlled lamp circuit - these are driven by the power driver board that generates the zero cross signal.
So, without looking at your machine and just going off the symptoms you describe, my guess would be a fault on the power driver board. Do you have any more information so I can try and narrow it down a little? If you have any test gear (e.g. oscilloscope), you could try probing the zero cross signal to see if it is active (it should be on pin 34 of the big ribbon cable that joins the power driver board to the CPU board).
Good luck.