Exception: "unique_object_service failed to handle..."

I last played Sims 4 three years ago and only installed the latest updates about a week ago. Whenever I load my family, there's an error that creates a new "lastException.txt" file (attached) with the main error message being:

unique_object_service failed to handle on_all_households_and_sim_infos_loaded due to exception (IndexError: list assignment index (0) out of range)

I already ran a repair and deleted the "avatarcache.package", "localthumbcache.package" and "Resource.cfg" files but none of that fixed it. There are no mods (not even objects or CAS stuff) and I didn't notice any weird behavior, apart from the error.

The exception only appears the first time I load a save after restarting the game. If I load a save, then go back to the main menu and load another save, it doesn't create a second exception file. I've got a couple of old saves and I tested each one: The only one that doesn't cause that error was created pretty much directly after the family had moved into their very first apartment after creating them (a long time ago!).

None of the saves caused any exceptions when I last played! What's going on here, what's causing the error and how do I fix it?