To get the codes for 2.8.3.4 :
1. Go to https://accounts.paradoxplaza.com/games and log in (or create) your Paradox Account.
2. Scroll Down.
3. Find "Access Earlier Versions"
4. Click "Show"
5. Go to your Steam Library and select Crusader Kings II
6. Right-click and select Properties.
7. Under the “BETAS” tab.
8. Enter the code for 2.8.3.4 retrieved from Paradox Plaza and click “CHECK CODE”.
9. Select 2.8.3.4 and wait for Steam launcher to finalise the switch.
10. If the previous step fails, exit the game, right click properties, local files, verify integrity of game files and wait for all files to be verified.

📣WARNING: If you have Holy Fury you need to both disable it in the Steam interface and the game launcher. Disable all other mods outside the WtWSMS collection.

For more details, feel free to visit our sub-forum.