So, did you get this error?

WARNING
The files of mod 'Divinity: Original Sin 2' and/or its
dependencies are invalid. This can be caused by corrupt files or
some files have been changed. You won't be able to play this
mod in multiplayer.

Nothing works, the Steam file validation doesn't do anything! It's still broken after a complete uninstall and reinstall! Just what do you do at this point?

Go to "My Documents\Larian Studios" and delete all applied mods
If it doesn't work, go to your game data installation folder in Steam and delete any mod files

First process removes all installed mods,
second removes local mods you might've made with moding tools.

Both are untouched upon Steam's "Verify Integrity of Game Files" option so this one can be quite confusing. Don't worry, all your subscribed mods will be downloaded again when you launch the game. So unless you have some mods you've been working on yourself or not subscribed to mods you're using for some reason, you shouldn't lose anything.