Common solutions for: Freezer defrost drain frozen
01 - Temperature Control Thermostat
A frozen freezer defrost drain can be caused by the freezer temperature being too cold. If the temperature is significantly below 0° Fahrenheit, you should use a multimeter to test the temperature control thermostat for appropriate changes in resistance as the thermostat cools down or heats up. If the resistance doesn't change or there's no resistance at all, the thermostat is defective and will need to be replaced.
Highly Likely
02 - Clogged or Freezing Defrost Drain
Defrost the drain and make sure the drain is clear. Using a turkey baster with warm water will help clear the ice and any clogs quicker. The water should flow freely into the tray by the compressor. If the drain is clogged the drain strap will not keep the water from freezing.