Rodi
We have been tryin to tackle this for a week at least and I am fuggin stumped, The Jesus is a Hort guy and not dumb by any stretch all the normal things one would point to seem not to be the issue.
Looks like a temporary lock out caused by something. When's the last time your PH pen was calibrated ? Did you happen to use something different to raise or lower your PH ? Have you checked the shelf life of your ph up & down ?

TJ you run a salt tank the shit is hard..