Okay. I'm sure it's obvious to everyone by now; 1.31 is the bane of the mod.

Every time you open Diplomacy, no matter who you play, it crashes.
But the previous version worked just fine.

https://www.moddb.com/mods/lord-of-universalis/downloads/lord-of-universalis-2-v210

Grab the mod there (Mod Database). Extract the contents to Documents/Paradox Interactive/mod.
Now, before you go and start the mod, not yet. Open "lou_m.mod" with notepad (or Notepad++), scroll to bottom and add your user name for your computer to the "path".
Now, switch to version 1.30.3.
Now open local files of the game directory, and open "Dowser.exe" (it'll be where the game exe is), this will open the launcher so that you can activate the mod.
Now with the mod active (may have to add it to it's own playset an enable it there, then select it on the main menu just below "Play" and "Continue"), start the game with "Play".

Best way to tell the mod is starting, you see the Mod picture for the mod instead of the standard EU4 an will hear LOTR music instead of EU4 music.

This IS your best option right now. Cause the latest update (1.31) makes the mod for the most part unplayable.

Enjoy :)