The cause is your career data: While the zone is loading (selected lot), the engine ‘instantiates’ your characters, and all of the objects within the region. It loops and validates all of the tuning and stored data, such as your sims’ stats, and inventories. If the engine encounters a corrupted data node, or field with invalid values, it exits the loop, and logs an exception. That’s a good way to prevent catastrophic failure, but it means that your household won’t load fully when updates or mods cause tuning conflicts, or invalid data to be stored within your sim_info. This happens most commonly with bad career data, and career data/tuning errors should have been expected with all of the Discover University career updates. Never save, until after you’ve tested all of your mods.

The only solution are to load another household, then cleanup the data with a custom script, or just roll back to your last good save. You should have at least five previous versions, which were automatically stored within your save directory.