Überdüngung

Have you measured the runoff yet? What is the EC value? I think it looks more like a phosphorus deficiency. Either due to too little or the wrong pH value of the soil. I wouldn't flush and would test this first. I've also had blockages or over-fertilization myself, and it looks a bit different. I also don't see any clawing, which is typical for too much nitrogen; the leaves would be dark green. And the spots your plant is showing look like phosphorus deficiencies.