For the sake of that super clean MPU board, PLEASE get those batteries off the board!!! Either a remote battery holder or consider NVRAM conversion. Not only can the batteries destroy the MPU board, it can also eat away at the connectors below causing the need to completely repin all of them. So, don't wait!!

If other's suggestions don't fix the issue, as was mentioned, I would suspect a bad LM339 on one of the Opto Interface board under the playfield. When strange problems in the switch matrix like this show up, its always best to change out all of the 339's on all the opto boards.

You may have overlapped one of the ribbon cables when you reseated (plugged bottom row into top row of pins ). Use a flashlight and recheck your work.

Left outlane? It's the right one that's brutal! That reminds me, I need to order the lane extender.