@Dreamer_Simmee Hello, There are numerous reported issues of Custom Content and Mods being broken with the latest update implimented using DX11, so now custom wallpapers are in need of batchfixes, thumbnails for some custom conent are either missing or have a questionmark like you are seeing, etc.
I would suggest moving all mods and CC out of your folders for the time being and playing a vanilla game temporarily.
If you know how to use the Sims 4 Studio, I would do a batch fix, as this has helped to recover most Custom Content. As for Mods and other Custom Content you may need to locate the creators for the new updates and fixes.