I managed to fix the problem using nhancer to tweak the witcher profile defined by nvidia. Each driver that nvidia release "includes" profiles for games currently on the market. With an adanced tool like nhancer, for example, you can access and modify these profiles. It's like the nvidia control panel, but with much more options available.In nhancer i force anti-aliasing mode using the option "Enhance in-game AA setting". Here are the explanations given by nhancer author for this option :"Usually, if you force an Anti-Aliasing mode through the driver, all rendering is done with the AA mode selected, regardless of what the application itself tells the driver to do.This works with most games. But especially more current games have an option to enable AA within the game itself and give the driver very specific information where to apply AA and where not.Enabling AA from within the game is more compatible and more efficient than forcing the AA mode through the driver, but has the disadvantage, that you're limited to the AA modes available in the game's options.If you enable the option "Enhance in-game AA setting", AA will only be applied as defined by the game - so you have to enable AA in the game as well. But the driver still controls which AA mode is applied, which gives you the best results: optimal compatibility and efficiency while still being able to use all the new modes offered by nVidia."I also forced 16x anisotropic filtering.The problem in game has almost disappeared for me. If you have a SLI system like me, don't forget to set SLI mode to "Alternate Frame Rendering", to uncheck the 09 flag and to check the 03 flag. That fixes the problem of sun and lightnings visible through walls.Anyway, even if we can fix the problem using advanced tools (for nvidia users), this very annoying bug has to be fixed by the devs.