I seem to be having an issue with rendering shadows in CS6. I've never seen this before and I can't see any settings that would cause it. It only seems to be relating to a single light, which was actually copied from elsewhere where the issue wasn't visible. I've also copied the same light into a different point elsewhere in the same composition and it has no problems.
Here's the same with the shadow intensity increased (normally at 20%, increased to 109% for the sake of making it more visible)
Thanks, I tried this but it doesn't seem to fix the issue. I tried it up to the highest resolution and it's still having the same issue just with slightly thinner lines. It's strange because a bit further into the same composition the problem stops. It seems to only effect a couple of layers.
Think of a shadow casting Light in AE as a sort of camera. Anything this Light "camera" can't see should render shadowed.
The most likely explanation for these artifacts is that you have two layers that are very close to one another in space. The light is unable to properly determine which layer is in front and which is behind (which is shadowing and which is shadowed). You can fix this by any of the following:
1) Separating the layers more until the artifacts go away.
2) Setting Casts Shadows to Off on one of the offending layers.
3) Setting Receives Shadows to Off on one of the offending layers.
In addition, these artifacts can become more pronounced as the path from the Light to the layer approaches a glancing angle. Imagine looking straight on to a piece of paper: you see a lot of it and in AE it would occupy a good amount of your shadow map resolution. Now imagine looking at paper from its edge: you see almost none of it and there is not sufficient resolution to accurately map shadows.