Dont have a schematic in front of me but if J207 is the plug leading to the playfield for these switches and when unplugged you get no error, than this proves the board is Ok and the fault is on the playfield.

Thats what I thought too, until I did the diode / alligator clip test as per Clay's guide with the testing of columns and rows....The same column (when tested that way) without playfield attached gives shorted error....

Me thinks it the U20.  Easy enough to replace and mine looks original and is very common to go.

I found a little batt acid damage too on some resistors too....Just a little green and fuzzy, so I cleaned with tooth brush and 50/50 vinegar / water.  Letting dry under heat lamp and wont be going back in for at least a week....