Check the switch itself. You can try just shorting the two pins coming out from the bottom of the switch with a screw driver where the wires are soldered to.
Next, try shorting the red wire on the switch to ground somewhere - the metal coin door should be grounded. If the switch works it indicates an issue with the yellow-red wire going back to the solenoid driver board on connector J2 pin 7.

The diagnostic buttons yellow-red wire goes back to the MPU board on connector J3 pin 1.
When you played with the MPU J1 connector to fix the display issue, you might have flexed the MPU board enough to get that J3 connector to make enough contact to work.
I suspect this problem may come back again and you might have to visit that J3 pin 1 for repair, especially so if the MPU board has battery corrosion reaching that connector.