The dust container/vacuum cleaner bag contains debris that smells bad.
Vacuum cleaner without vacuum cleaner bag: Empty the dust container.
Vacuum cleaner with vacuum cleaner bag: Change the vacuum cleaner bag.
Tips to prevent unpleasant odours:
Do not vacuum up liquids or any damp dirt. Allow steam-cleaned or shampooed carpets and rugs to dry out completely before vacuuming.
You can also equip your vacuum cleaner with the Active AirClean filter. The active charcoal in the filter neutralises smells before the air is released to the environment again.