The FilterBag contains debris that smells bad.
Change the FilterBag.
To prevent this problem, make sure that you do not vacuum up anything wet. You can also equip your vacuum cleaner with an “Active Air Clean” filter. The active charcoal in the filter neutralizes smells before the air is released to the environment again.
If you wish to store the suction hose separately, the patented Miele Hygiene Cap is available for this purpose. The cap is placed on the appliance instead of the suction hose so that neither dust nor smells can escape from the FilterBag.
