Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Very generally the E1 error means that the sensed speed is wrong somehow - probably not sensed at all.
Given that the treadmill runs at low speeds, but errors out at higher speeds, I advise you to check whether the magnet-to-speed-sensor separation is a bit too large. It needs to be set to a lesser separation. The proper separation of the sensor from the magnet is ideally about that of the thickness of a credit card.
It may be that it is at a marginally large separation where it can sense the speed if it is low, but the 'signal' from the magnet is too faint when the speed is larger and the magnet whips by faster.
Let me know how that goes, adjusting the sensor to be closer to the magnet when it passes that is. (That's done by loosening the sensor's clamps, sliding it closer to the magnet when the works have been turned (with power off) so that the magnet is just opposite the speed sensor.)