Graphics card drivers are completely up to date.
Operating systems have been Windows 7 and Windows 10.
Graphics card is a GTX 660M
This happens in multiple different games. WoW, Archeage, H1Z1, Runescape, and more.
Take a look at the below screenshots and you'll see exactly what I'm talking about.
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My comment below shows more images of this.
Best Answer
You are using a NVidia card.
It probably is not a hardware problem as shadows are not 'special', so the issue would appear on any texture.
There is a big chance that it is Ambient Occlusion. Go to your NVidia Control Panel, open the '3D' menu and find the 'Ambient Occlusion' option. Turn it off. That should sort the issue out.