Inadequate rinsing after descaling.
Do you need descaler? Delonghi's EcoDecalk has been specifically designed with your machine in mind. Using this descaler regularly will prolong the lifespan of your machine and keep it running at optimal performance.
De'Longhi Coffee Machine Descaler EcoDecalk 500ml Best Value Pack (delonghi.com)
Rinse as specified in this step-by-step guide: