Ferroli RVL - Error 41?

Just moved into a new build property, we have a Ferroli RVL external boiler.

It keeps flashing up Error 41 on the unit. You press the manual reset buttons and it will function normal (ie heating the radiators etc) for approx 20 to 30 mins then it will Error 41 again. the process repeats itself regardless of how many times the unit is reset.

Does anyone know what's causing the Error 41?

Any help will be greatly appreciated.