How can I fix this?

http://prnt.sc/c4e013

How can I fix this?

Something similar can be seen with ReShade off but with it on it's a lot more visible.

More screenshots:
ReShade on
http://prnt.sc/c4e2ml

Off
http://prnt.sc/c4e3rd

What you see there are banding artifacts because the color range to display a smooth gradient is not enough, the steps from one color to the next possible one (like RGB 100 100 100 to RGB 100 100 101) is too large so one color step goes on for many pixels. Luckily the ReShade framework has a shader called "deband" that basically blurs out and dithers those areas without touching other areas and hurting details. Base configuration is just fine, enable it and done.

Tried it and it doesn't make a big difference. There is a slight improvement but the banding is still there.

Then try to configure it. I managed to remove all banding from my Witcher 3 with it but don't remember the exact settings unfortunately. Also make sure that the deband is last in the pipeline and applies after all other shaders (except image grain or things like these).

There's also a deband for the SweetFx shaders and another from a different shader author with the one from SweetFx as I understood it mostly solving potential shader caused banding from the injected shaders whereas the other one is a adaption from one of the movie players (Media player classic?) and from my own testing it has worked really well in most games although it does introduce a slight blurriness.

I used dither from sweetfx and it works much better than deband. It doesn't remove banding completely but now it's unnoticeable unless I'm specifically looking for it.

I configured it and managed to remove pretty much all banding but the image quality worsened significantly. I'm using "dither" from the sweetfx shaders and it works great. Now I can only notice banding in the night sky and only if I specifically look for it.