Diagnosis
The internal drain pan is overflowing. This happens when the unit is running in a very humid environment and the auto-evaporation can't keep up, or the drain path is clogged.
How to Fix It
Unplug the unit. Tilt it slightly back (so the back is a bit higher than the front) and check if water drains out from the bottom. That's the quick fix to redirect water to the evaporator. For a permanent fix, locate the rubber drain plug on the back or bottom of the unit. Pull it out, let any standing water drain into a pan, and leave the plug out if you're in a high-humidity area. You can attach the included drain hose to that port and run it to a floor drain.
If That Didn't Work
The drain pan or internal drain tube might have a crack or be disconnected. You'd have to open the housing to check, which is a more involved repair.
Heads Up
Never run the unit with the continuous drain hose attached AND the drain plug still in place. That'll guarantee a leak. It's one or the other.