Freezer ice forms from moisture that gets in: humid air condenses and freezes on cold surfaces, and the water that products carry follows the same path. Danfoss's cold room guides list air infiltration, high humidity, poor airflow, wrong defrost settings and drain problems as the causes of icing.
The cause can usually be read from where the ice sits. Heavy ice near the door points to frequent openings, and frost around the door to a worn gasket. Thick ice on the evaporator means defrost is too short or too rare, or the drain is blocked, because if defrost does not melt all the ice, more builds up on the next round. Ice around the product points to product going in warm, wet or unwrapped.
Ice on the evaporator is the expensive kind: it reduces airflow and makes temperature unstable. Manual defrost is done before the ice thickens and the product is moved beforehand; a sharp tool can pierce the fins and pipes. If the ice returns fast, add the photo, the temperature curve and the defrost data to the service request.