Flashing FLO means the switch is reading open when it should be closed. So when the spa sets circulation to occur, it is not reading that circulation is actually occuring at the flow switch.
Can you take a video of your flow switch when circulation is turned off, and then back on to see how the paddle in the switch behaves?
Something you can try to do is with all circulation off, take your filter off, then take garden hose and force water from the hose through your circulation plumbing. Start with pushing water from the filter side, then push water through from the jet side where the heated water comes out. This may flush bubbles or any obstructions out. If you can't get a good seal with your garden hose, try using a clean rag and put it around the area where the garden hose meets the spa. You should feel cold water coming out of the other end - if pushing from filter side, cold water will come out from jet where heater water comes out of, and vise versa. You may also initially see larger bubbles come out, this is good and means you had air trapped in the plumbing. If you don't feel the cold water come out something is obstructing the flow. At this point, you can try using a good heavy duty (and clean) toilet plunger to push water through.
Is your spa heating up to set temperature even through the FLO error?
When you cut power to the spa, and turn power back on and immediately run your jets, do you still get the error?
From my understanding you replaced the original flow sensor that gave you the FLO error with a new one, and new one still gives you same flow error? If so it's highly unlikely it's the sensor.
Is your circulation pump same specs as old one - production enough flow to completely push close the paddle in the sensor?
Last thing it may be (and worse case scenario) is the circuit board malfunction.
