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Most/many times this is a clogged up or restricted defrost drain system, but once in a while the drain system has been cleaned and everything appears to be good but you still get ice in the defrost drip tray and such and the drain freezes over. Some manufacture designed kits to help and some home made remedies.
Defrost drain strap kit, prevents drain freeze-ups
Defrost drain tube kit with instructions
Sometimes a solid ground wire wrap around the defrost heater and the other end stuck in drain hole has helped for me. Avoid letting the wire or whatever to touch the plastic side of the hole.