If on MAC: Disable the Steam In-Game Overlay for Crusader Kings II
If on Windows, try a clean re-install:
* Important: Go to '..\Documents\Paradox Interactive\Crusader Kings II\save games\' and copy all the save games you want to keep outside the folder
* Uninstall Crusader Kings II
- Uninstall the game from within steam by right clicking on the game and select "Delete content"
- Go to the folder where you installed Crusader Kings II in, and check if the Crusader Kings II folder is removed. If not, delete the entire folder
- Installation folder can be found at: "..\Steam\SteamApps\common\"
* Remove document files
- Go to "..\My Documents\Paradox Interactive\" and remove the entire Crusader Kings II folder
- Re-install the game