For those who are still having this issue and in case it helps one or two of you -- since it could be any of a number of script mods -- in my case it was Meaningful Wedding Stories by Rex/konanhurry. The mod has since been updated.
Luthienrising's advice is spot-on, as always. If you don't know what's causing an issue, remove a number of mods (the Occam's Razor or 50-50 method is the most efficient), load a game, and just keep going till you can narrow down the file/mod that's causing the current problem. Yes, it's a little tedious -- especially if, like me, you have a lot of mods/cc and save files take a while to load -- but it's better than possibly damaging your 9th-gen legacy save (or *gasp* having to play without mods). XD
Because I'm autistic and can't help explaining things: If you're not sure what this whole 50-50 business is about and don't use a mod manager (like me, because I've had bad experiences with mod managers in the past so now I do things manually), the WHY of doing it is because that method is statistically quicker than removing mods one by one (or removing them all and adding them back in one by one), unless you get lucky. There's a bunch of ways to organise doing it, but here's how I do it:
1. Set up a 'testing for broken mods' folder somewhere; some people use the desktop, I just use a folder in my documents (but not in the EA/Sims 4 folder). Because I'm also ADHD and have trouble being methodical (and have a ton of mods), I actually use sub-folders in there for each batch of mods I've moved out of the mods folder. They are cunningly-named Batch 1, Batch 2, etc.
2. Move (not copy) about half the mods in your Sims4/mods folder into the testing folder.
3. Load your game, and a save. If you get the error, the problem *wasn't* with the mods you moved. Quit the game. Leave the moved mods in the testing folder for now.
4. Move half the mods remaining in the Sims 4/Mods folder into the testing folder.
5. Load your game, and a save. If you still get the error, repeat steps 4 & 5 until your game loads and doesn't throw the error. Now you know that the broken mod is in the last batch you removed.
6. Add half the mods in the 'threw-an-error' batch back into your mods folder and load the game. If you get an error, it's one of those mods -- which, by now, should be a pretty small possible selection, so it shouldn't take too many more iterations to find the culprit. Rinse, repeat until you find the mod that needs to be either removed or updated.
Once everything is pinned down / updated / cleaned up, move the mods from the testing folder back into the Sims 4 / Mods folder and you should be good to go.
In my case, I removed *only* script mods (and their associated packages, if any) because others had already discovered that script mods were the issue. I also kept MCCC, TMex's Better Exceptions, Lot51 Core and a few other 'core' scripts in my mods folder because a) they'd been updated already and b) I was 99% sure it wasn't them, and they are usually helpful in tracking down problems (or required for many of the mods to even function). I didn't remove my CC for the same reason: no scripts.
One final note with apologies for the pedantry to those who already know all this: consider organizing your mods folder if you don't already. Having 1200 cc, package, and script files just lying around in there makes updating and maintenance a giant PITA. Just remember that script files can't be more than 1 folder deep inside the mods folder or they won't work (so e.g. Sims 4 > Mods > This folder can haz scripts > This folder can't). And if a respected modder like Twisted Mexi tells you to install their mod in a particular way, do it that way even if it might not really *have* to be that way -- because 100% they know better than a player how their work should be installed for the least possible issues. Anyway, my mods folder has a ton of subfolders, each one labelled with the name of the modder and the mod (or mods): e.g. LMS (Little MS Sam) More Woodworks, Adeepindigo Education Overhaul, or MCCC Version blah.
Hopefully this will help someone. If not, I got to practice my typing.