Please try a full clean re-install:
STEAM
- move any valued save games elsewhere
- "delete local files" in Steam
- manually delete all files left under Steam/CK2 AND Documents/CK2
- re-install game in Steam, run a verify when done.
GAMERSGATE
- move any valued save games elsewhere
- uninstall the game from Windows
- manually delete all files left under the GG CK2 AND Documents/CK2 folders
- re-install game from GG but point it to a new location outside the system path, such as c:\games\ck2\
- apply the latest patch (only)
Now, before you copy your save games back in, click on the Save games tab. Assuming it's now OK, move your valued saves in one by one - probably one is corrupt and crashing the game.