That looks still like it could be nutritional or Mareks as the birds are of different age groups. It's not possible to tell strictly from a video unfortunately. Incidentally, natural light is important in the absorbtion of calcium. D3 is made from another D vitamin via interaction with light. Keeping them in a darkened place could have effected their D3 manufacture causing rubber legs and rickets.
Have you started the nutritional aspects of this treatment yet?
Have you started the nutritional aspects of this treatment yet?