Re story here
In my use of solder it’s a blob of conductive metal that fuses electrical connections together as they are held fast in the solid metal of the solder junction. How is mere dampness going to cause a milspec quality solder joint to fail structurally? Beyond this how is “mold” going to effect the metal joint?
Not sure about damp conditions, but as manufacturers were moving away from lead based solder, tin whiskers were becoming an issue. I retired almost 5 years ago and haven’t kept up. Perhaps that’s what they mean by corrosion?
Galvanic corrosion.
But, it’s really a stretch…
This is a stretch but the real problem may be corrosive flux attacking the copper wire near the solder joint. I restore antique radios from the 1920’s - 1930’s and this IS seen sometimes when “plumbers flux” with zinc chloride was used.
But I can’t imagine that any aerospace electronics would use plumbers flux or that whatever flux was not cleaned off after soldering. 
Antimony in the solder may promote corrosion, so the error may be the use of the wrong type of solder… solder that contains antimony (instead of lead. )
Cecil said similar conditions in the Mir space station led to a fungus, which produced “corrosive chemicals such as acetic acid”, which “pitted Mir’s titanium, plastic, and glass”. Would an acid that damages those things also mess up solder eventually?
A few guesses, some of which have already been mentioned:
Dendritic growth. Conductive dendrites can grow (via electrochemical migration) between two solder joints when moisture is present, causing shorts. The risk of dendritic growth might depend (not sure) on the solder alloy and the type of flux used.
Galvanic corrosion (moisture + dissimilar metals). This is not usually a problem with electrical solder joints, though.
Flux contamination. Perhaps they did not adequately remove the flux residue after soldering. Any inactivated flux that remains on the board is highly hygroscopic, and could cause electrical shorts. But this is more of a flux & cleaning issue, and not the solder alloy itself.
Tin whiskers. If lead-free solder was used instead of tin-lead eutectic, tin whiskers could be a problem. But unlike dendrites, the presence moisture is not necessary for tin whiskers to grow.
The solder itself may not fail, but the presence of a joint vs. an unbroken conductor means there will be some less-protected conductor on either side of the joint that will be slightly more susceptible to corrosion. I see this pretty frequently in boat wiring.
I understand but in this case they re talking about (it appears) the composition of the solder being inadquate so assumedly the metal solder itself is susceptible to be attacked.
“Okay, guys, we are building one of the most sophisticated and expensive weapon systems on Earth, intended for one of the most demanding purposes yet conceived. Somebody run down to Radio Shack for some solder. Oh, wait… Home Depot’s closer.”
That’s meant to be funny. Probably isn’t, not to anyone who’s read postmortems on systems failures of these types.