I actually just ran into this issue which was quite weird. I remember that the last time I played my household, I was able to save just fine. On top of that, I just created this "vanilla game" and haven't played into it that much as I've been re-adding in all the lots and sims that I saved to gallery from my old sims 4 folder. The lots have all been added but the relationships are still a little screwed thanks to EA's complex inability to fix them most of the time in my case. Anywaaaays, neither here nor there... I went in my game today to try to fix my relationships, again, wanting to get my family back to a point where it doesn't feel I had to start over (though it still feels like it does). Anyways, my household loaded and before I pressed play on the UI and before getting the error, I used the cheats that I found on SIMSVIP to "hopefully" fix the relationships since for some reason in cas my teen twins cannot be made "daughter" or "son" to their Adult aged parents. The only options are brother and sister when I try to set the relationship and it's the same even if I were to age them down to children. -_-
Anyways, the cheat worked for the one twin but not the other. Not sure why. I tried twice. I figure I'll build her relationship up with her dad since they're acquaintances RN and then try to do the cheat again later, if, after building their relationship doesn't trigger my original attempt to work so it would say he is her dad. So, I moved on to fixing their social groups. New names, new do's and don'ts etc. My point? For 2 of my sims groups, I did use Zerbu's custom group icons. After I did all that, I attempted to save. That's when I got the error. So, I'm not sure if the error had anything to do with that or the cheat codes or not but the lot and everything on the lot was able to save the last time I played and I haven't added anything new since then and those cc icons by zerbu I've had for a bit. I tried switching lots as suggested to my twin sim's half sister's father's household. Saved the game and it worked. Not sure what the deal is. Seriously. Will doing that just cause my game to become corrupt anyway over time? I always save my games as "save as" and I did have a last exception file. It basically mentions relationship tracking and other relationship text and the sort. Probably bcus I messed up on typing in those sims id numbers when placing in the cheat codes. Not sure.