Can't get replacement thermostat to work.
Hi, I'm trying to replace an existing Tado Smart Thermostat with the same model, as the original one keeps depleting batteries. I thought it would be a simple job of just removing the old one and putting the new one in the existing socket. I've done this and registered the Thermostat and the Tado app can see it in the system. However, although I can see it in the system and the temperature reading does seem to be correct, it's not turning the boiler on when required. When I press the button, the Tado display shows 4 vertical lines and and E, followed by an explanation mark. I can't find this particular error in the documentation but see attached.
I should add although this is a 'new' thermostat in that it's never been used, I've had it several years and the firmware is V20.7. Thanks for any help. Chris.