The pressure switch is a little round thing that attaches somewhere in the piping with a few wires running to it. Mine is located on the heater. It's one the back side, and hard to get to, but I'd start looking for anything on the heater tube to find the pressure switch. That switch is what tells the motherboard if the water is flowing. If it's bad, and the pressure isn't enough to pop it, then it'll relay a message to the motherboard.
When you did the shut off and restart, did you do it every time you tried a new step? So take out the filters, reset, then see what happens. While they are out, do the next step and reset. See what happens. You may have more than one issue.
If your spa is like my Catalina, which it sounds like it is, then you have two large pumps, and one turns on low to circ the heated water. Look for the heater (right under the motherboard), trace the piping to the motor to see which one circulates the water. Then, in the back of the motor, right in the center where the shaft would be, there is a little cap on the back of the motor. Take a screwdriver, pop off that cap, and insert a tool inside to spin the shaft on the motor. If the motor spins freely, then it's not locked up. Check for power to the pump, and see if it's getting 120 volts. If everything checks out, and you're still hitting the error, I'd try switching the pumps. When mine went, I just switched them out. It is a pain since you have to empty and refill the tub. When you do this, you may create an air lock too, so be mindful of that contributing to the flo message upon boot up. If after switching pumps and refilling it works, then you know the issue is with the pump. Before I would do all this though, I'd replace the pressure switch. It's like $40 bucks or so, and switched out rather easily.
My knife valves were bad, so even when I took out the pressure switch, I ended up draining the tub. I broke the old switch too, so I had to remove the heater to get it all back together.
Another thing is to check for grounds. I heard if the grounds are bad or not connected, the motors may not turn. Sometimes corrosion builds up on the contacts and can mess things up.