over 2 years ago - AndrewT - Direct link
If 1.5.1 crashes then this is not the known issue with 1.6 . Has 1.5.1 worked on this machine before?  
over 2 years ago - AndrewT - Direct link
Okay then, leaving it on 1.5.1 Please do a full **clean** re-install:

- move any valued save games elsewhere
- "uninstall" in Steam-CK3
- manually delete both the Steam/SteamApps/common/CK3 AND Documents/Paradox Interactive/CK3 folders
- re-install game in Steam, run a Steam Verify when done.
- add CK3.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test

In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.  
over 2 years ago - AndrewT - Direct link
The game used to run under .1.5.1 okay before 1.6 came out, I presume? If so I have no idea why it is not working that way now, that makes no sense. This is not the first report of that happening
forum.paradoxplaza.com

CKIII No longer starts

Hello, I have came up with an issue that totally prevents me from starting the CKIII game, from the Steam app, the Paradox Launcher or directly with the ck3.exe file found in steamapps/Binaries I have tried to: -Reinstalling probably 10 times... forum.paradoxplaza.com forum.paradoxplaza.com
Hopefully the patch to 1.6 will come out this week and fix everything!