J1 goes to the playfield
J2 goes to the backbox

Isolate where the problem is by adding J1, then test the voltages - still good ? No = There's a short somewhere on the playfield

Do the same with J2

Something is dragging the power supply down.

I suspect a problem with the Solenoid driver board IF the power is dragged by adding J2.

Simple process of isolation, just follow the basics with this quick test and report back !