That looks like a shadow bias problem to me.
I've seen figures before for it somewhere when I used to use Poser for rendering.
IIRC you may have to set it really high to get rid of it, typically 5 or 6, then again IIRI it could be the other way [lower], but you already have lower setttings, so I'd go higher!

Injustice will be avenged.
Cofiwch Dryweryn.