DIAGNOSIS The container's blade assembly seal has failed. The gray silicone gasket is worn, cracked, or displaced.
FIX Unplug the blender. Remove the container. Unscrew the blade assembly from the container using the provided wrench or a large coin in the slot. Remove the old gray gasket. Clean the groove in the container and the blade assembly. Press a new gasket (part # 11536) firmly into the groove. Reassemble and tighten firmly.
IF THAT DIDN'T WORK The plastic container itself may have a hairline crack in the threaded area. Hold it up to a bright light and inspect carefully. If cracked, you need a new container.
HEADS UP Overtightening the blade assembly can warp the container and cause leaks. Hand-tighten with the wrench, then only a slight additional turn. Check the gasket every few months for food debris.