Day 50, 43cm, 45cm - still too small, but flowering. I suppose this small size is due to pH problems. I checked the earth and water pH. Tap water pH is too high, earth pH is fine before adding water. It seems I can solve this by using destilled water instead of tap water. I already changed the water, but this will probably not change anything for the moment, due to the latency of my watering system.
I will start the next grow directly with destilled water and see if this helps.