Well it appeared to be a calcium deficiency, I got some special calcium additives and added a teeny weeny itsybitsy bit of it to the nutes. The problem seems to be contained: no more new leaves are affected by the brown spots and the plant is growing healthily again.

(Still in the middle of a heatwave here, 33 degrees centigrade outside! Unbelievable compared to the moist and humid summer we had last year!)

:thumbsup: