What causes the machine to lock the ball is switch 47 "Ball Popper" in the caster's club. The switch that you press when the issue happens is switch 42 "Right Boat Entry". Now they are both on row 4 , which typically points to a switch matrix issue. Most likely you have a shorted contact (diode or wire) that creates a return path for that switch 42. The first things to check are all the diodes and switches on the same rows as these 2 switches involved (both row 2 and row 7 are to check).

Row 2 has the following switches : coin door (22), left boat exit (32) , right boat entry (42) , center jet bumper (52) , Top right loop (62)
Row 7 has the following switches : trough 2 (17), left stand up target (27), reel 1 (37) , ball popper (47) , left slingshot (5-7).

A switch matrix issue requires another switch to be closed somewhere in order to produce the issue. Row 2 : Coin door is usually always closed , which means if its the culprit , your problem should occur all the time. Left boat exit is pressed momentarily, right boat entry the same , also center jet bumper and top right loop. So the chances that one of these switches is closed at the same time you hit the right boat entry is nearly impossible.

Row 7 : trough 2 is closed when you have a single ball in play so if it happens on the first ball but not only once there is one ball locked, then this shouldn't be it. left stand up target is momentarily pressed, reel 1 is closed when a ball is locked , ball popper is momentarily pressed as well when the ball enters casting club, left slingshot is pressed only when the ball hits the slingshot.

So it appears the first place to check is trough 2 and reel 1. Reel 1 is detected with an opto which are known to get dirty / faulty. After testing the diode and the switch for trough 2 (and making sure the wiring and diode placement is correct , ESPECIALLY if you have changed any switch in your restoration , it is VERY possible a new switch doesn't have the same pinout as the old one) I would then check the opto to see if it's dirty and in the switch test to see if it reacts properly. If both these components are good , I would go on every single switch in both rows to make sure the diodes , switch and wires are all correctly installed. I would also look at the connectors on the main PCB to make sure nothing is damaged or pinched anywhere along the cable.

Another thought on the reel itself. If anything is out of line or the wrong belt is used (even the wrong replacement belt could cause this) it's possible the reel is misaligned and that reel 1 switch closes. (This would also require a shorted diode on the opto board to cause your issue though).

Most likely it's a shorted diode OR wrong wiring of a new part with a different pinout. If you enter the switch edge test there should only be 4 closed switches , trough 1,2,3 and switch 24 which is "always closed". Edit : my coin door was opened , so if you close the coin door , switch 22 will be closed as well

Edit 2 : A quick test would be , place a ball in the reel using the casters club during a game and then enter the switch test menu. Make sure the other 2 balls are drained and that there is a ball in trough 2. Press the right boat entry switch. Does any other switch (ball popper) close at the same time ?

The switch mentioned by bigehrl is switch 32 which gives it a high chance of being part of your issue if it's diode is damaged or wiring is wrong or shorted from impact like he mentionned, and one of the switches I mentionned would be closed..