Why did my oscilloscope hookup trip my RCD?
The oscilloscope's probe ground is connected to the earth (0V). It's likely that the "ground" of the power meter is not really ground. It's likely that the board's "ground" is actually at the neutral or line voltage, so it would create a circuit between neutral and ground or line and ground, which the RCD detected. But, without a board schematic, it's difficult to debug.

To debug the circuit, a differential voltage probe would be best. Otherwise, the probe ground could be connected to the earth prong in the power meter.