That is a fairly high temperature for the outside coil to be freezing up, especially as it was only on for 20 mins, remember as well that the ammount of frosting will also depend on the ammount of humidity in the outside air.
The exterior coil in heat mode will always freeze up when run at low temps with high humidity, and is designed to go into defrost mode automatically every hour.
That one of yours probably is low on refridgerant, as it is freezing up to fast and at too high a temp I would recommend that the system be serviced
BTW for comparison my 4 year old outside coil down here in the humidity capital of the world seldom freezes up unless the outside air temp gets around the low 40's with higher humidity.
Cool Gerry you slipped in here while I was thinking about what to say trying to keep it simple to understand