Larian Studios Forums Divinity - Original Sin Divinity Original Sin - Mods [Main Campaign Mod] All Skillbooks Purchasable

I was wrong, this is the culprit:

If the Skills are not working, simply close the game and open it again.
This is because you need to have this file

C:\Users\_USERNAME_\Documents\Larian Studios\Divinity Original Sin\PlayerProfiles\_PLAYERPROFILENAME_\modsettings.lsx

Divinity: Original Sin creates this file when you load a mod for the first time. If you have never played a mod with Divinity: Original Sin then the game will not have that file. This causes the Skills to not work properly.