In order of operation, I always double-check PH. Then if PH is good I refresh the nutrient solution.
Your problem there looks to me like a P def, which is rare and suggests either incorrect PH or contamination. It could also be magnesium which is also likely if you have an RO system. Both would be fixed by fresh nutes and correct PH. Check your PH, if it is over 6 it could be the cause of a P def, if it is under 5.8 that could be the cause of a Mg def.
If that does not fix the problem then your problem is not a lack of nutrients, nor a lack of availability from incorrect PH. If that is the case, iron may be leaching into your nute solution.
Once you have eliminated the most common causes, check for any metal parts or other possible sources of contamination. After my experiences in the last year, I would no longer have any metal parts in a recirculating system in contact with the nutrient solution.
I am unfamiliar with Hunter solenoids, sorry. Maybe find some technical specs or a datasheet.
IME parts designed for run to waste flow are often unsuitable for recirculating systems.
