You can completely disconnect the 20V (both positive and negative) from the USB board (the fan will

You can completely disconnect the 20V (both positive and negative) from the USB board (the fan will not work then though) - that will break the GND loop.
Or 2. remove the GND finger on the mains plug/or the earth wire from the mains lead that connects to the power supply input/or use a mains cable without grounding.
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