The reason for the E81 error message on the hot tub may be due to a faulty RCD, base not paired with the control panel, large distance between the control panel and the control base unit, a low charge on the control panel, or other malfunction in the motor part of the hot tub. Test the RCD first. If you have pressed the TEST button on the current protector, you have de-energized the base. Turn on the RESET (RÜCKSETZEN) button again, this will restore the power supply to the motor base. The wireless control panel could've been showing E81 because it was running on its own rechargeable battery while the motor base was without power.