Deserialization Error? Easy fix without going to the CRINGE beta patch
step by step tutorial to fix your contraptions not loading. also im assuming you are on windows, because im on windows, idk if it works on other things,
find this file and back it up, it is your contraptions file, this is what you want to protect, win+r
C:\Program Files (x86)\Steam\steamapps\common\People Playground\Contraptions
now we need to go to a version before this bug came to be, the most recent one is aug 9, steam doesnt easily give you it, but you can get it via the console. win+r
steam://open/console
now the console is open, enter
download_depot 1118200 1118201 7435198814107509496
this takes that recent version and downloads it on a separate folder, here. win+r
C:\Program Files (x86)\Steam\steamapps\content\app_1118200\depot_1118201
now we need to go to this deposit and find the contraptions folder, go back to the one saved on your desktop and copy it, and replace the empty one in the depot,
run the application in the depot and use that till the absent father that is devoleping this dying game fixes it.
thats all cya