Light leaks perhaps. I had a grey container like that and it does not keep out all of the light ( I also used FF nutes). Wrap your container with foil and use hygrozyme, at least thats what I did and no more Root rot. FF nutes + DWC light leaks+ temps above 72 = Root Rot.
Im not dogging FF, I just think you need to be extra cautious with them in DWC.
I did recently switch to House and Garden and its even more impressive. Their best product is the Roots Excellerator. Use it with hygrozyme and your root rot days are over.