[2023] Game crashes loading with exact same modset used day before with no issues

It's possible the mods have done something new and different to that save file, due to difference circumstances in the campaign.

Can you start and play a new test campaign with them okay? If so probably best to take this to the User Mods forum.  

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 - In particular make sure there are no files or folders left under either the Steam CK3 or Documents CK3 locations, before installing again.
- 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
- if that works, then enable ONE mod and test
- etc

I hope that does it for you!  

That sounds like that mod is doing something to the base game installation, which it shouldn't!