Aha, very interesting. Prosperity 1.8 works, and successfully starts a game where everyone has a standard starting deck. And if I "cheat" the starting deck to instead be 5 Coppers and 5 Estates, then Prosperity 3.20 loads, but Base 1.2 doesn't. So it seems fair to guess that they're validating whether your starting hand is something you could have achieved using zaps, based on the cards you're supposed to start with?
Okay, sweet. We can basically make the levels fair then, though it's more complicated. Zaps can turn Shelters into Duchy or Curse, and Curse to Estate, but can't turn things into Shelters.
Do each side separately. Count the number of treasures.
- If it's 7, make them all Coppers and make the other cards Estates, except leave Shelters alone.
- If it's 6, make them 5 Copper and one Silver, and make non-Shelters Estates.
- If it's 5, make them 3 Copper and two Silver, make non-Shelters Estates.
- If it's not 5-7, leave it alone. Dunno if this is relevant but no need to risk anything here.
If it's obvious where it is in the file, I would also set free zaps to 0, because hey, the idea is to make levels fair and enjoyable, not for you to always have a 2-zap advantage.
We (you) could balance the 5-Estates advantage by giving the opponent Duchy-Estate-Estate; it's potentially more complicated than that, because the opponent might not have a starting Duchy (and you can't zap your opponent up), so I wouldn't worry about it. I guess also sometimes it may give you Curses, and we could keep one and have 4 Estates 1 Curse. Same deal, does not seem worth the effort.