Unbelievable. I just ran into this problem on my BSD. I just got the machine to work on and was having difficulties on getting consistent results.

I would push the Enter button and it would work sometimes and not work other times. Then when I finally got the Switch Test to work, the Up and Down button would not work. I finally was convinced that all the buttons were not working at the same time. Something common?

My solution was to check the ground wire (as you already did) and resolder the black wire to the first switch (Enter button). Since all four buttons are out, it has to be the common element (ground). I understand that you metered it all the way back, but perhaps it's not a solid connection?

I also Scotch-Brite(ed) the connectors on the Coin Door Interface board to ensure that the connectors were making a good connection.