Diagnosis
Mold and biofilm in the hidden parts of the machine: the rubber door gasket folds, the detergent dispenser drawer, and the internal sump. Using too much detergent or fabric softener, and always closing the door, creates the perfect damp environment for it.
How to Fix It
This is a multi-step clean. First, pull out the detergent drawer completely. Soak it in hot water and vinegar, and scrub the cavity it slides into. Second, wipe down the entire rubber door gasket, pulling back the folds to clean the grimiest part at the bottom. Third, run the proprietary Drum Clean cycle (or the hottest, longest cycle you have) empty, but add two cups of white vinegar or a washing machine cleaner—not bleach—to the drum.
If That Didn't Work
If the smell persists, the issue is deeper in the plumbing. Check and clean the drain pump filter (as debris there rots) and consider running a cleaning cycle with a specific biofilm-removing washer cleaner, not just vinegar. Also, ensure you're using High-Efficiency (HE) detergent and never more than two tablespoons per load.
Heads Up
Never use bleach in a cleaning cycle if you've just used vinegar—the combination creates toxic chlorine gas. Pick one cleaner. And the single best preventative is to leave the door and detergent drawer open after every single wash to let everything air dry completely.