Bosch KGU Fridge/Freezer alarm cycling

A 7 year old Bosch fridge/freezer KGU400670EV has started to periodically display "AL" on the freezer temperature display. Any and all help very much appreciated.

I have checked the evaporator behind the cover inside the back of the freezer - no ice.
For much of the time a thermometer placed in the freezer section shows -19C, the same as selected temperature, but after about 20 hours, the "AL" is displayed and the thermometer reads more like -3C. After about 4 hours the "AL" disappears and on checking the max temperature experienced (as per the handbook by pressing the cancel alarm button) it too shows -3C. This cycle then repeats itself so after another 20 hours, (during this 20 hours an occasional thermometer check shows -19C), the "AL" is again displayed.

it seems to me likely during the defrost cycle the temperature inside the freezer section rises considerably, but why??

All suggestions most gratefully received.
John