A cracked or 'dry' joint is just that.  There will be a hairline crack in the solder itself or sometimes a bit of a hole.  Whilst they don't look to be anything nasty, they can and often do render that particular pin defective.

I keep coming back to the point earlier that the machine started working a for a few balls or games after you re-seated the connectors.  That is a very good symptom that the connector/s or header pins are your problem.  Gav is spot on and there could be other components that have failed but the first step is to rule out the connectors and pins as your problem.