Diagnosis
The self-evaporation system is overwhelmed, or the internal drain path is clogged. In very humid conditions, the unit produces more water than it can evaporate out the exhaust hose, and it overflows the internal pan.
How to Fix It
First, ensure the unit is perfectly level front-to-back and side-to-side. Use a small spirit level. If it's tilted, water can't flow to the evaporation area correctly. Second, check that the exhaust hose isn't sagging or looped upward, creating a backpressure that hinders evaporation. Keep it sloping gently downward if possible.
If That Didn't Work
You'll need to manually drain it. Place the unit near a floor drain or on a towel, remove the internal drain plug (usually a rubber stopper at the back-bottom), and let the pan empty completely. Re-plug it. In sustained high humidity, you might have to do this every couple of days.
Heads Up
Don't ignore this leak. Water inside the unit can eventually trip the full-tank safety sensor (even if the bucket is empty) or cause electrical issues and mold. Regular draining in humid spells is part of owning a portable AC.