Exactly ^.
Didn't we already cover this in that other thread? The scope probe ground reference leads are all connected together at the scope chassis. So you have to use a _differential_ measurement on the output of the diode bridge, where you do not connect the two output channel probe ground reference leads at all, because the input measurement probe already has its ground reference lead connected to one side of the input to the bridge and you don't want to short this to the output side through the probe reference leads.
So you use two scope channels + Math subtraction to obtain the output measurement. Evidently the OP has a good 4 channel scope so making this differential measurement should not be a problem, and in fact it's a good learning exercise.