To fix mods and custom content try the 50/50 method:
1. Move all the mods and cc out into a temporary folder on the desktop.
2. Move a few files back at a time. Load the game to make sure it loads and everything is good.
3. Delete the localthumbcache.package from the Sims 4 folder.
4. Repeat steps 1 - 3 till there is no more files to check.
** While your testing, don't save the game. If you find any problems, delete the mod or custom content file.
How To Find Problem Mods and CC - http://simswiki.info/wiki.php?title=Game_Help:TS4_Finding_Problem_Custom_Content
Broken/Updated Mods: Python/Terrain Update + Get Famous, Nov. ’18
Custom content can become outdated with most new game patches. If the game works when the Mods folder is removed, then one or more of the mods and/or custom content has become outdated and will need to be removed or replaced. EA/Maxis aren't responsible for broken mods/cc. It's up to the creators to fix.
January - last edited January
Just wanted to say make sure that you did not accidentally add a sims3 mod, that will cause this problem.