Bit late in the day but I had this error 2 years ago on two similar Sapphire Spa hot tubs (mine and my mums) which failed within 3 months of each other. I traced it to the impeller on the pump which had sheared on the vanes. Both hot tubs approx 7 years old at the time. Pump ran and everything looked normal then thermal cutout after 60 seconds as no water was actually being pumped through the heater. Fixed by buying a replacement head unit for the pump - approx £80 and I also changed the shaft seal for £6. I suspect the shaft seal failing allowed water to the "gubbins" and caused the impeller to seize/snap. Given the parts are all plastic and the 7 years of chemicals it's no surprise the impeller failed.
