Go to Documents/Paradox Interactive/Crusader Kings II. In there, delete the folder called "gfx". Note that this is NOT the /gfx/ folder under Steam/SteamApps/common/CK2 .
Now restart your game, it will recreate that folder and fix the problem.
You also need to add ck2game.exe to the exceptions list/whitelist of your antivirus app.
Obviously disable all mods until they get updated accordingly.