The impact protection (bumper) is heavily soiled.

Clean the impact protection (see figure) with a soft, dry, or slightly damp cloth.
The robot vacuum cleaner returns immediately to the base station. The robot vacuum cleaner moves out of the base station, then drives round in a circle once and returns to the base station. This action may be repeated one more time.
Clean the impact protection (see figure) with a soft, dry, or slightly damp cloth.
Place the base station on a hard floor.
If the fault persists, please contact Miele Service.