Diagnosis
The hose isn't creating a continuous downward slope from the unit to the drain. Gravity drain requires a steady decline; any upward loop or flat section will cause a backup.
How to Fix It
Disconnect the hose. Ensure the dehumidifier is elevated at least 2 feet above the floor drain or sink. Reconnect the hose, making sure it runs straight down with no dips, kinks, or horizontal runs. Test with a bucket under the hose end first.
If That Didn't Work
The internal rubber drain plug might not have been removed when you installed the hose. Unplug the unit, remove the bucket, and check the drain port inside the bucket compartment—the rubber plug must be taken out.
Heads Up
Using a hose longer than 10 feet or with a small diameter (less than 5/8" ID) will severely reduce flow and cause backups. Use the recommended hose type.