The coffee machine door was opened during the descaling procedure without the user being prompted.
Or the brew unit or containers were removed.
Start descaling procedure again and follow instructions on the display.
For details of the exact descaling procedure, refer to the operating instructions for your coffee machine.
A description of the procedure for descaling various coffee machines can be found in the article “Entkalkung starten und durchführen (Kaffeevollautomaten)”.