Hi uj
I do not know - I am guessing and asking questions and hope: "can narrow down on the problem(s)".

Your pin was functioning (well, You have always played a 4-player game). Starting a 1-player game You could play to (3rd) last ball. You have lost the ball - questions: Did the pin counted down the Bonus-Ladder to Zero ? For a short moment You had "Game-Over Light lightet in the Backbox" ? Then the pin started itself a new 1-player game. You played that game - ball-1, ball-2, ball-3 and now my same questions (Bonus-Ladder ? GO-Light ?). You played many such games - in the end You gave up - so You had to toggle-off "in the middle of a game" ?

By now, You cannot start a Multi-Player-Game - pressing the "Replay-Button" does nothing (?).
Please locate the Coin-Unit (usually next to Ball-Count-Unit, usually in the cabinet) --- can You manually step-up the Coin-Unit to 2-, 3-, 4-player game ? and the play ?

What exactly happens at end of last ball, 1-player-game ? Bonus-Ladder ? GO-Light ? Does ONLY the Reset-Relay pulls-in (and let the Score-Motor run) ? Does the GO-Relay trip and latch-on just afterwards ? does the Coin-Relay pulls-in ? does the Credit-Relay pulls in ?

I made a JPG using the ipdb-schema (may look-up there) --- in a functioning pin starting a NEW game (therefore starting a 1-player-game) happens: You push green-3, current can flow through green-1, -2, -3, -4, either "-5 or -6 or -7 or -8", so "-9" pulls-in and its moved plunger closes "-10" so "-11" pulls-in, closes "-12" and let "-13 the Score-Motor" run. THEN the Reset-Procedure takes place and finally the ball is kicked over to the Shooter Alley.
During "Reset" are "RESETTED": Coin-Unit and Player-Unit and Ball-Count-Unit. When You now want to start a game for 2nd, 3rd, 4th player: "green-6 is OPEN, -7 is OPEN, -8 is OPEN" --- current can ONLY flow through "still closed green-5".
So for Your problem "Cannot start a 2,3,4 player game: I would look at Coin-Unit --- when the Unit is stepped up: A rod/stud mounted on the wheel OPENS a switch when stepping 3-player to 4-player. Maybe a wire has broken-off there ?

Starting a new game for 1 player does some more stuff: red-1 does close, because Game-Over-Relay is TRIPPED at the time - red-2 is CLOSED (at that time), current flows red-1, -2, "-3 pulls-in" and establishes "Self-Hold-Current through -4, -5, -6, -7). The Score-Motor runs, so through -8 the -9 is pulsed ( to reset the Score-Drums). Score-Motor runs, so through -10 and "pulling relay / moved switch-red-11": red-12, -13, -14 are resetted.
When starting a game for 2,3,4 player --- red-1, red-10, "not pulling relay therefore not-moved red-11": red-15 is actuated (Coin-Unit steps-up to 2,3,4 player).
So for Your problem "pin starts automatically a new game": I am tempted to investigate on red-1, red-2, red-4. BUT I HAVE PROBLEMS: You can play "normal" ball-1, -2, -3 ...

And so I come back to my many questions (see beginning of this post). Greetings Rolf

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