Diagnosis
Biofilm and mildew are trapped in the hidden areas of the machine: the rubber bellows (door boot) gasket and the detergent dispenser drawer.
How to Fix It
Run a cleaning cycle with a washing machine cleaner or two cups of bleach on the Clean Washer cycle with hot water. Then, manually clean. Wipe the entire interior of the rubber bellows, pulling back the folds to expose the inner lip where debris collects. Remove the detergent drawer by pressing the release tab and soak it in hot water.
If That Didn't Work
Mold can grow inside the drain pump filter housing. Clean the filter as described for the F21 error. Also, leave the door and detergent drawer open after every use to allow the tub to dry completely.
Heads Up
Using more than the recommended amount of HE detergent, or using regular detergent, creates excess suds that trap moisture and soil in the system, accelerating mildew growth.