I was able to start a game earlier immediately after patching and buying the DLC. I played for a bit, closed the game, and when trying to open it later it crashed and gave an error message before the splash screen.
Verifying game cache didn't work, re-installing the game didn't work.
Running Windows 7.
The only fix seems to be: (1) un-subscribing from all the Steam Workshop mods, and (2) deleting the mod files in the User\Documents\Paradox Interactive\EU4 folder.
However, I did not try either just (1) or just (2), so I don't know which one is sufficient, but doing both seems to work.