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Sounds like drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan.

I believe the drain tube attaches to the drip tray (Item 4 in Section3) under he evaporator coil. Though some units drain into a funnel like part at the top of the fresh food section and then out the drain tube.

I would check that the drain tube is clear.
If plugged a turkey baster with warm water, an air compressor with low pressure or a wire can often be used to clear it.

Also check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes.

Be careful that the drip tray does not overflow and empty the drip tral after you are done.

Be sure to unplug the unit while doing this.

Drip tray is under the evaporator coils in the freezer (Item 4 in Section 3).
Drip pan is under the unit in the compressor compartment (Item 16 in Section 1)