The cutout you made in the third picture, for light protection, is it very light-tight? If too much light is getting through either at the cutouts or on the perimeter, it could indeed cause the problem you are having.

I would still tell you to add an active form of aeration for your nutrient solution. My best guess on your problem is inadequate oxygenation of the roots. Fork out the $10 for a pump at wally-world and another $4 for some air stones. Put those in your reservoir and let them run 24/7.

Another thing you might try, if you have any more plants not attached to the nutrient film, is to remove one plant and set it on a plate out of the system. Water it a couple of times a day to keep the rockwool moist and see if the roots do any better.