Try playing completely without mods, to make sure there aren't any issues related to the mods themselves. If it still crashes and you have a specific save that is crashing, you can share it here and I'll take a look at it to see if it crashes for me as well.
Otherwise a complete re-install usually does the trick:
-Uninstall the game
-Go to the "\Documents\Paradox Interactive\Crusader Kings II" folder
-Copy any save games you want to keep
-Delete the entire "Crusader Kings II" folder
-Re-install the game
Otherwise a complete re-install usually does the trick:
-Uninstall the game
-Go to the "\Documents\Paradox Interactive\Crusader Kings II" folder
-Copy any save games you want to keep
-Delete the entire "Crusader Kings II" folder
-Re-install the game