Yes, the jumper is definitely not supposed to be there. See pic below of my Playball. The wire insulation is dirty, but I can see the green w/red wire.

Sounds like you indeed might have a high-resistance path somewhere in the circuit to energize the Ball Return solenoid.
You are confident that the ball return solenoid itself is firing strongly if you jump 24V to its terminals?

Sounds like you have checked the normally open switches on the O relay and at Motor 2C. Have you cleaned/adjusted the normally closed switch on the 3rd chute relay (W) that is also in the circuit to the Ball return solenoid? It is the NC switch with black & red and brown & red wires to the switch tabs.

Lee

IMG_3151 (resized).JPG

Could the coil of the U relay be shorted (and thus drawing most of the current)?
Might try unsoldering the wires to the U relay and see if anything changes.

Otherwise, the bad wire or a loose solder joint between the ball return solenoid, the O relay switch, the W relay switch or the motor switch 2C. Wiggle all the wires and make sure one isn't loose at the solder point.