Unfortunately it seems there’s no silver bullet answer – this sort of artifact seems to be part and parcel with modern dynamic shadow techniques. I think you’ll find if you crank your Shadow Bias (on your stationary light) way up, it helps significantly (note the slider only goes up to 1 but you can type in higher values). However that tends to make objects look like they’re floating, which you can somewhat offset by turning on Contact Shadows. Tweaking dynamic shadow quality is a bit of a tightrope act.
I have noticed that really large objects tend to suffer the most, so I’d recommend making sure your world/landscape scale is reasonable (using a human scale reference).
Sorry I don’t have a more satisfying answer, good luck!