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This is certainly your AV app preventing write access to that folder. A new ck2game.exe is not recognised as okay by it. Perhaps @bestbrian can help you with details for Avast.

Thank you Andrew, I tried turning off my AV entirely last thing yesterday rather than just trying to create exceptions and that for reasons I'm not entirely clear on, worked. I was able to launch the game through once, turn the AV back on whilst ingame, shut the game and then restart it with AV on and no problems.

Thank you again for your help and I hope if anybody is having this problem that my solution fixes it for you too.
 
Please do a full **clean** re-install:

- move any valued save games elsewhere
- "uninstall" in Steam-CK2
- manually delete both the Steam/SteamApps/common/CK2 AND Documents/Paradox/CK2 folders
- re-install game in Steam, run a Steam Verify when done.
- add ck2game.exe to the exceptions list/whitelist of your antivirus app
- start the game with no mods active and test

In particular make sure there are no files or folders left under either the Steam CK2 or Documents CK2 locations, before installing again.


Also if you have installed RadeonPro, or a gaming VPN app called Evolve, or the MSI Gaming App, or Duet, disable it as it clashes with this game.


IF NEITHER OF THOSE HELP:
DXDIAG is a program you run from a command prompt or the Windows start menu 'run' dialog box (or 'search programs' in Windows 7 or later). 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 your Documents/Paradox/CK2/settings.txt.
From your Documents/Paradox/CK2/logs/ folder, attach system.log .

Had this problem and confirm this fix works. It's important to delete all folders after uninstalling the game.