No real idea, it is not at all a known problem with the game.
Has it always happened since you've had this game? Or it was okay up to some point?
Does it matter if the saves are local or in the Steam cloud?
Definitely no mods in use?
DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach here your Documents/Paradox Interactive/CK2/settings.txt
From your Documents/Paradox Interactive/CK2/logs/ folder, attach here system.log , error.log .