Well, the first picture has roots that look perfect and the second picture looks like they've become desiccated. What is the white material/background in the second picture?

How do you apply your hydroponic nutrients? Does the NFT mean nutrient film technique? Your problem has to be one (or more) of the following: light contacting roots, insufficient oxygen in your nutrient solution, pH of nutrient solution, chemicals in nutrient solution, or pests.

Could you guess on the nutrient solution temperature? If it is too low it will shock the plants, too high and it will not hold the necessary oxygen.