Just to be sure, here's what I mean by a full clean re-install:
- move any valued save games elsewhere
- "delete local content" in Steam-CK2
- manually delete all files and folders left under Steam/CK2 AND Documents/Paradox/CK2
- re-install game in Steam, run a Steam Verify when done.
- try game with no mods active
If it worked before on this same machine, there's no real reason why it wouldn't do so again once reset back to the new state.