The cause is unknown.
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 Miele Service.
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 accessory pack.
If necessary, you can order more silicone grease from Miele Service or the Miele online store.