Those wires need to be soldered to the switch as well.

Like this. Green to right lug, white to lower left. Make sure your banded side of diode is at upper left.
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Do exactly what Chris is saying. If the error goes away you have a playfield issue. If it doesn't go away you have a board issue.

If it goes away you have to check for a short on the playfield. A switch is shorting against something. Look for things that don't look factory like sloppy soldering etc. That is most likely where your issue is as operators hack up games most of the time and don't care if they play perfectly. Specifically check all switches on row 1 as you error message referred to. Gun trigger (11), slam tilt (12), Left ramp enter (31) (make sure opto solder joints aren't touching metal protector plate), trough 1 (41) (most likely not it), Wall target down (51)(area of trouble on a lot of shadows because it's a drop target), left sling (61), mini left standup (71), mini right standup (81). Check manual for switch location if you can't find them.

This is a pic of my shadow. Unplug these. Then plug them in one by one to see when error happens starting from left to right. They are in the lower left of head box.
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Yes, it's on playflied. Make sure a diode isn't touching both lugs. There is a short somewhere, not a bad diode. Well, I diode can be bad but that shouldn't cause a short. Look at ever switch on row 1. Anything in cab that could touch a switch with playfield lowered?

I once found someone soldered a set of wires to a gi bulb holder and later cut them off . They most have done that to power something else. Part of the cut wire was touching the other part of lamp socket. This wouldn't cause switch issues but look for some garbage like that on the switch matrix. All switches should be wired the same. Check for one with a variation.

Which set off connectors creates the short when you plug in, green or white? What "j" connector?

I had to just replace my opto board. Most likely you hit it when changing the flipper bats.

Yeah, I'm debating whether to send your way for repair or just sell it as is.