It may be coincidence that the stand up targets stopped registering after you fixed the lamp board. Go into switch test and manually push each target, first with your finger, then use a ball and roll it at each target. Do they register either way?

Also just found this for you...

https://pinside.com/pinball/forum/topic/t2-how-to-troubleshoot-stand-up-targets

Awesome news! Don't forget to mark this thread as solved.
Enjoy your game.

I wouldn't recommend messing with the board. Unless you happen to have years of experience with working on PCB's, you might do more damage to the board than you fix.
I would try a new connector first, if the header pins were a bit rough the female connector may be tarnished as well. If you are not concerned with keeping it original, converting to Trifurcon connectors is a good way to go.

After that, I would check the targets themselves, the contact points may be worn down too much, a diode may be failing somewhere in the chain, both of which could lead to screwy behavior.

Any luck yet?