Yeah, I checked that, and the gap seems fine. It looks like this:
I would have thought that if the board were shorted out, it wouldn't work with any power supply.
(In case my initial description wasn't clear, the only difference between my working and non-working scenarios is what's on the other end of the USB cable - either a PC USB port or a phone charger USB port.)
I am tempted to add a resistor to my power supply voltage to reduce it a bit, and see if it works better that way, since that's the only externally measurable difference that I can detect. Hmm