Hilfe! Weis nicht mehr weiter? Nährstoffsperre!?

Moin, we don't know each other :) My name is Falko, and I'm really desperate right now. I've got a sudden and severe deficiency in my plants for the umpteenth time in the 4th week of flowering. Since it's not clearly recognizable, I'm concluding it's a nutrient lockout. The pH of the medium and the watering water in