Thermostat not switching boiler on

ipr1

I've had my tado system in my new build for 5 years now and it's been fine. This morning I changed the batteries on the second floor thermostat and now it's not switching on the boiler to heat that floor. The thermostat display is fine and the app is fine. All systems say they are fine and connected.
It clicks to initiate the boiler, but it doesn't respond.
The ground floor thermostat still works, so I'm a bit stuck.
Any ideas?

Emcee

Hello @ipr1,

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