Buderus Ecomatic error

I have a 2 zone system, with a dedicated DHW circuit. During the summer mon ths the system works fine, but as soon as winter arrives and the system sta rts supporting the in floor heating zones it gets a period Err that leaves the system circulating the DWH pump but the boiler does not start.
To clear this I need to hit the reset button, the boiler immediately fires and the system runs normally for a few days until the Err reappears.
There is some correlation with this error to moving from summer operation, DHW only to winter operation supporting the heating circuits.
Is this a configuration issue, or a controller malfunction?
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