Please do a full **clean** re-install of Steam and game:
- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- exit the Steam app entirely
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox Interactive/CK3 folders
- delete everything in the Steam folder EXCEPT steam.exe and the SteamApps folder
- run steam.exe to rebuild all that
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test