You may have bent a terminal under the playfield. Start by narrowing down where the problem is by disconnecting the playfield 1 point wire at the Jones plug.

I re-rubbered a '77 Big Deal and had the same problem. I got some thin cardboard (cereal box type) and put pieces all the way down in the (in my case) 10 point switches. When I started a game, nothing was locked on. I proceeded to pull 1 piece at a time until I found the one that locked on again. The contacts were not touching and I didn't see anything from below. The contact was being made by two blades as they went through the playfield. After a little bit of adjusting, the problem was gone. Just an idea...

Does the game fully get through its startup? Or is it still trying to find 0?

If the reel is at 0 when you start the game does it happen?

Also check the other ends of the switches where the wires are soldered onto the tabs. Often a stray wire end or a bent tab can make contact there to complete the circuit and not be easily noticed. Similarly, some switches had stiffener blades that could short to the adjacent blade when the contact faces were open and again not be noticed.

Post some pics of the restored game. Riverboat is a great game with a lot going on!

Solder splash on the side of a switch stack will also short a switch closed.

Well if it was me, I would seperate those contacts for those two switches marked in blue and try to plug that jones plug back in.

Also check in the female end of the plug and make sure there is nothing inside causing a short.

Nice looking game!