claimME It sounds like you might have a mod conflict, or the broken mod is dependent on a different mod and won't load (and therefore break the game) unless that other resource is present. In that case, if the two mods were tested in separate groups, each group would be fine on its own.
The way to test is to add mods without removing the ones you've already tested. When you add a batch that causes the game to stop working, you'll know that you've found one of the problem mods, and you can split and test just that batch with all the other mods present. When you find the one problem mod, test the other mods against that, as in, leave the broken one in and add and remove other mods until you find the one that conflicts with it.
As for why the new save works, it's very possible that one of your mods only breaks things when some feature it includes is added to a sim. For example, a custom career or trait would likely only be a problem when assigned to a sim.