Thanks Vlad...you're a genius!
Check ths following Low nitrates, Hard water, Chlorine content in the water, Aphids or Thrips, possible high pH
Also this looks beyond a deficiency, there is tissue damage beyond a mere chlorosis...
I'm no expert, but I just learned that if you're growing in a greenhouse and have fruit/veggie-bearing plants growing together with the leafy plants, the ethylene gas emitted by the fruit/veggie bearers for ripening can cause brown/discoloring spots on your leafy plants. Might this be your cause?