would need to see more.. looks a bit lush, but i'd still wager caclium deficiency. 40cm is probably a bit close, though... each plant can be a bit different. if node spacing is proper then the light distance is fine. Could potentially be too high with K, causing a Ca lockout, too. or, maybe pH is a bit off which can cause anythign and everything -- which usually means more than 1 symptom shown in the case of pH issues.
not enough info though.