The inside coil of the A/C unit has ice on it when the temperature outside is cooler than 65 degrees. Freon has been added, but it has iced up again six months later. What is the problem?
If just an AC unit that makes sense to me unless set up to make a "cold room" depending on the type of control (capillary tube, orifice) the system may not be able to accurately meter at cooler outside temps as there is no real feedback compared to a system with a throttle valve TXV. With open loop the Freon is compressed and cooled when it expands being close to 20 degrees colder ice will form if there is not enough airflow across the evaporator or the delta of tempature is two low. If trying to make a cold room a shifted temp sensor needs to be used to controll the system and get to lower temps.
Most A/C units do not work well at temperatures below 65 degrees. Years ago, we would install a low ambient control. Ask the HVAC people that service your unit to install a "low ambient control device" to help keep the head pressure up to allow the unit to operate below 65 degrees.
Have a tech check the high and low pressures of the refrigerant before the ice has formed. Low refrigerant pressure (as from a leak) will cause the temperature of the evaporator coils to be below freezing. You want the indicated pressure to correspond to a low side temperature to be 37 - 40 F.