There is a serious bug in The Witcher 3 which can break the game even when one scripted mod is installed. With the following method you can actually start the game with scripted mods.

Installation

• Unpack ScriptCompileFix.7z into the The Witcher 3 Wild Hunt\Mods folder. This folder can be found where you installed the game. If you don't have the Mods folder, create it.
• Copy the Mods.settings file into Documents\The Witcher 3 folder.
• You have to insert a record for every mod you want to use into this Mods.settings file. You can use any text editor for this.
  
  

Example

This is the original Mods.settings file:
[modBaseScripts]
Enabled=1
Priority=9999
[<insert the next mod name here>]
Enabled=1
Priority=9998

Let's say you want to use a mod that makes Geralt look like Batman (and this mod goes into the following folder: The Witcher 3 Wild Hunt\Mods\modMakeGeraltLookLikeBatman). 
Then you have to modify the Mods.settings file to look like this:

[modBaseScripts]
Enabled=1
Priority=9999
[modMakeGeraltLookLikeBatman]
Enabled=1
Priority=9998

If you want to use more mods append their data at the end of the Mods.settings file. It is important that modBaseScripts has the lowest priority (highest number) so it will load last. The lower the Priority the sooner a mod is loaded in the game.

Uninstallation

Delete The Witcher 3 Wild Hunt\Mods\modBaseScripts folder and everything you added to the Documents\The Witcher 3\Mods.settings file.

Thanks to CD Projekt RED for creating this great game.
Thanks to skacikpl for making a guide which this fix is based on.