For most of the measurements, you can use either the primary or secondary ground. For measuring anything on the 2184, you should place the ground probe on the negative rail.
That was for a battery powered scope or multimeter. You cannot ground a mains powered scope to any supply voltage. To view waveforms with the negative rail as the reference with mains powered scopes, you need to use the scope in differential mode.
I don't have anything on this specific board. Boards with a similar layout of the two op-amps have a triangle waveform on pin 1 of the bottom op-amp. I was hoping that you had the same here.