ALU = At floor leveling unit.

Not sure if that's relevant though, I think it's only used for group controllers.

When you replaced the backboards did you reuse the little voltage regulators that are mounted on the side of the metal cage and wired to the boards? If so, did you confirm you have a clean 14vdc coming out of them. There should be 2 capacitors on the right side of the cardrack you can measure the voltage across fairly easily. Also, flip to AC and see if there's any AC coming through.

Those can go bad and cause all sorts of input issues. Usually the card leds will stay on or off and it will be a row or entire card but I wouldn't be surprised if it could cause this issue.