Solid flow indicates a "bad flow switch" that does not "open" when the water stops, and the circuit board is giving you the resulting "Solid Flow" message. You can confirm this by removing the sensor loom from the circuit board causing the circuit board to recognize the removal to be an open switch, and when you put it back on immediately after, the open was recorded by the circuit board and the spa will operate properly, that is until the flow switch is supposed to open again, but doesn't.
Replace the flow switch.