Remove every board and connection from the PSU.
Don't power the scope PSU! Only test with DMM. Luckly the short will be gone. Otherwise it's in the PSU!
Then connect each board, each connector, only one at a time. When the short appears, follow that connector/board.
Use thick wires in the bench PSU output, or the power will be lost in them.
If at 1A you find nothing, increase to 2, 3, 4, 5A...
You want to avoid burning any track, so give 1 minute between each step, so the shorted part has time to heat up.