I get this same issue and the error.log in documents/pdx/CK2/logs says
[virtualfilesystem.cpp:711]: Could not open file: gfx/flags/flagfiles_0.tga, error: Access is denied.
This apparently is a reference to the gfx folder in the documents folder, not in your steamapps folder. I tried running Steam as admin and removing the read only restriction from the CK2 folder. Did not help.
I also tried running the ck2 executable as administrator which was unsuccessful as well.
I'm not sure about the updates and when my steam has downloaded any, I don't recall having any updates today (16th May) but the game was running fine yesterday.
EDIT:
Tried deleting the documents/CK2 folder (Assuming that's the cache everyone's talking about? I don't have a folder called Cache anywhere

) and I'm still running into the same error.
If the log is correct it sounds kinda funny that CK2.exe creates a file into the cache but doesn't have a read access to it..
EDIT 2:
Now that I'm feeling helpful, here's the following DLC packs I have:
Holy Fury
Reaper's Due
Rajas of India
Sons of Abraham
The old gods
The republic
Sunset Invasion
Legacy of Rome
Sword of Islam
No content packs, I believe the other stuff comes with the DLCs such as the first 3 dynasty coat of arms packs and a bunch of portraits.
Also reinstalled the game through steam but didn't remember to check if the local files were actually all deleted.