[2023] Can't save my CK3 game, need help

We can't help with modded games here I'm afraid.

If you both start a new campaign with NO mods at all enabled, do you still get this?

If so does it affect both local and cloud saves?  

There is no logging of the game's attempts to write a save, by default. You could try enabling debug mode though.

I don't know anything about the structure of its save files - you might try user Mods for that.