Cause not known.
Replace the damaged sealing ring. A replacement sealing ring is included in the scope of delivery of the coffee machine. If necessary, you can order new sealing rings from the Miele Customer Service Department.
To prevent future damage to the sealing ring:
Clean the connecting piece regularly with a damp cleaning cloth without wiping the grease off the sealing ring.
Grease the sealing ring regularly once a month with the silicone grease supplied in the coffee machine’s companion pack.
If necessary, you can order more silicone grease from the Miele Customer Service Department or the Miele online shop.