Your water temp looks fine to me & with in the target range.
How ever they are other disease which may effect the roots perhaps the use of H202 may help out or other supplement.
I'm reasonable sure high PPM doesn't effect roots tho.
On the off chance you might of over feed to cause nutrient burn ?
It could be the plants are drinking lots of water & not taking up enough nutrients this will adversely effect the PPM value of nutrient solution in the dwc bucket etc