Diagnosis
The rubber gasket (sealing ring) on the blade assembly is either missing, cracked, or not seated correctly.
How to Fix It
Unplug the unit. Unscrew the blade assembly from the jar. Remove the black rubber ring. Clean the groove it sits in. Inspect the ring for cracks or nicks. If damaged, replace it (part of blade assembly 61095). Reinstall it snugly in the groove before screwing the blade back on.
If That Didn't Work
The jar itself could have a hairline crack in the threaded area. Hold it up to a light and check carefully. If cracked, the jar needs replacement.
Heads Up
Do not run the blender without this gasket. Liquid will leak into the motor base and cause a short circuit.