IMHO, I think
@trippingballs is right on the money.
It's my opinion that you're overfeeding.
The lockout is a micro. Probably calcium. More likely too much, than too little.
It's why they're improving on a fresh reservoir change too. Then once the plants get what they need, and are left with an abundance of what they don't, they go back to the way they were.
If you've been topping off, between changes, it would only magnify, your problem.
Excess calcium tends to cause rust spots, starting from the vain outwards, and affects the fans.
All your filters for your RO, are working fine? I'm assuming you check before adding nutrient.
IMHO cut your ppms. After the flush.
Maybe cut them by 30%, and go from there.