This may or might not be the cause of your issue... but I've discovered that running on low disk space while playing or updating while disk space is low can cause error code 801. Initial loading of the game seems to go well and then switching to another household or going to world mode or CAS and then trying to load back into a household becomes unsuccessful with the infinite loading screen or crash to worlds view with an error showing. Some lots are also missing and invisible. I first noticed this after the release of For Rent.
So, make sure it's not a low disc space issue and if not, check if it's a mod issue by moving your mods or Sims 4 folder to your desktop or somewhere and testing without mods. If the problem is resolved when testing without mods, try the 50/50 mods test method. Continue to test until the problem reoccurs and and then narrow down those by taking a small amount of mods out testing repeatedly until the problem goes away. Once the problem goes away, you can narrow down the specific group taken out that made the problem go away. This is how you can at least find out if it's disk space, mod related, or a game error. If it's a game error, most likely it can be fixed by a game repair or fresh install.
I hope this helps, but if not, I wish you the best on getting your game fixed.