When the foreground lights are at the minimum setting, there is a couple of small patches where the grey base colour shows through at the edge of the stones. This is very obvious on the screen if the plants are removed totally, however it is faded in the screendump.
Try setting the resolution to 1280 x 1024. That is my default for GA and it even increases the problem area (see image). However in N518.0.9.1618 Prolific fixed it by lowering that grey area far beneath the gravel.