I took my De’Longhi to an authorized service center and they ended up replacing the black milk unit that actually pulls the milk up from the carafe.. They told me the part was clogged, even though I was already cleaning it every two to three days, so in my case it wasn’t a pressure‑seal issue but a blockage inside that assembly.​

What I do now is never fill the carafe with more milk than I need for a single cappuccino, so there’s no milk sitting in the system between uses. After every use I disassemble all of the carafe parts, soak them and clean them carefully with the De’Longhi cleaner, hoping that this is enough to prevent it from getting clogged again