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@TheSwissOpossum

There is possibility, that files in your game folder or even in the Tray folder responsible for loading objects to your library are corrupted so you need to get fresh ones to eliminate this possibility.
Step 1. Rename The Sims4 folder to "The Sims4 old"
Step 2. Fix the game in Origin
Origin/Library - right-click on TS4 - option "Repair game"
You have to wait until Origin will fix all packs, not only the base game.
Step 3. Turn on the game - a new game folder will be created and you will have two - one old, one new
Step 4. Check if you still have this problem in the newly created game folder. If the problem disappeared:
Step 5. Delete in the new game folder following folders:
"saves" and Tray.
Step 6. Go to "The Sims 4 old" folder and copy - your "saves " folder.
Step. 7. Paste this folder in a newly created game folder.
Step 8. Do the same with Tray folder and TrayDeleteItems file
Step 9. Check if there are newly updated version mods you are using.
Then copy and paste your mods and CC from the old mods folder to the new one. First copy the scripts mods every time checking if the game is working properly. If you have a lot of CC try 50/50 method ( divide your CC into smaller batches for example 50 pieces in one subfolder and check these subfolders one by one)
Step. 10. Launch the game and check if this problem also disappeared in this old Tray folder However, it may happen that although the new Tray will not have this error anymore, the old Tray will remain damaged.
The last step - delete The Sims4 old folder.

Your saves and library will be not changed or damaged

I don't work for EA