Okay I quess I found a solution; in sc_lights_sunShadowEdge.fx find the line half2 blurAmt = (5.0f/shadowmapSize)*rand.xy;
Now change it into something like this half2 blurAmt = (5.0f/shadowmapSize)*rand.xy*200;

My quess is that this blurs (or maybe pixellates) the edges of shadows more hence why it removes (mostly) the black lines.

And I see now that sc_screen_default.settings.lights.sunPssmSplits is better to be left at 4, otherwise shadows a bit further away can look weird.