Since it is happening at the lowest leaves only, you can eliminate a few things right away. Deficiencies in mobile nutrients show in lower leaves first. If I had to pick one, itd be Potassium. That would explain its stunted growth, that plant is very small for 7 weeks.
Youre feeding a relatively high EC. Deficiency due to lack of a nutrient seems odd. A lockout may be happening due to an excess of one element or environmental conditions.
What are your temps during lights out? Is the floor really cold? I've seen similar results from temp problems.