Diagnosis
Food and mold trapped in the seal groove. This is a maintenance task, not a defect.
How to Fix It
Remove the blade assembly. Pop out the silicone sealing ring. Use a toothpick or dental floss to scrape the now-exposed groove in the plastic. Wash all parts in hot, soapy water. A small brush helps. Dry completely before reassembling. The ring must be dry before reinsertion.
If That Didn't Work
For mineral deposits or stubborn stains, soak the plastic blade unit and the ring in a 50/50 water and white vinegar solution for 30 minutes. Scrub again.
Heads Up
Never put the blade assembly base (where the metal drive socket is) in the dishwasher. Water ingress will rust the socket and damage the bearings. Hand wash only.