From the support FAQ topic in the Technical problems forum:

Quote
Problem
Cannot change difficulty because the slider is missing or invisible.

Solution
Quit the game and then remove this file:

Windows Vista and Windows 7: C:\Users\[WindowsUser]\AppData\Local\Divinity 2\Profile\settings.dv2

Windows XP: C:\Documents And Settings\[WindowsUser]\Local Settings\Application Data\Divinity 2\Profile\settings.dv2

If you cannot find the files, make hidden files visible. After deleting the file, restart the game.

To set Windows Explorer to show hidden files and folders: XP, Vista, Win 7.