One more thing, there seems to be ~44VAC between earth (either scope probe's ground or middle (green) terminal on psu) and both negative and positive terminals
Is that normal/ok?
44VAC sounds like either you're measuring between earth and a floating ground or your powersupply is damaged and is outputting unregulated 44Vac.
Have you checked all the powersupply voltages with the multimeter after the issue? There are 7 measurements to make.
-SIDE1-
GND <-> + (should be half what display says)
- <-> GND (should be half what display says)
- <-> + (should be exactly what display says)
-SIDE2-
GND <-> + (should be half what display says)
- <-> GND (should be half what display says)
- <-> + (should be exactly what display says)
-SIDE3 FIXED -
+ <-> - (5v)
Are you sure the powersupply green terminals are mains earth and that none of the fixed output 5V terminals are grounded?
The only way i can see for you to have poped a breaker would be if you connected the scope probe ground to something which had voltage with respect to mains ground. But it would have had to be pretty high voltage/current to draw enough to pop the breaker. I dont see how any powersupply voltage (5v/12v) could draw enough current from the 230v side to pop a breaker. Its a bit of a mystery.
Maybe a good idea to check the mains plugs on the powersupply/scope are wired correctly (if they've been changed or are using a detachable cable)