Thanks for the advice!
With the machine off, I have measured the resistance of the solenoid (No. 4 "Subway Popper") and compared it to other, working solenoids. They all have the same value (appr. 10.5 ohms).
With the machine on, I have measured the voltage of the solenoid and compared it to other solenoids (again, the same value).
I have not yet tried to "ground" the solenoid because I'm not quite sure how to do it. I guess that the door should be closed (in order to get high voltage to the solenoids) and that a connection should be created between ground (e.g. the power metal box) and one of the solenoid lugs (the one with only one cable connected?).
I also did a switch test on switch No. 44 named "popper" in the manual (is that the switch you are referring to?). When I use the switch test menu, No. 44 works when I move it with a plastic pen. However, if I lower the playfield, put a ball in the trunk, let it roll to the trap door, the switch is NOT activated according to the test menu. Could this be the problem?