Krups 8108 brewing flask only halfway down

When the machine is switched on, the brewing piston automatically performs a test run. With my machine it only goes down about 1 cm, then stops and goes up again. Then the two lamps CALC and CLEAN flash. The machine only reacts to the flush button. Then steam comes out of the milk nozzle for about 5 seconds, all lights start flashing and the machine stops responding. This process is repeated exactly the same way, no matter how often I try to switch it on and off.
The curiosity: It is my second machine with exactly the
same problem.

I've read the problem with these two flashing lights on the net before but never found a solution. It seems to be an error code for a certain cause

.
Anybody got an idea?