The pulsation steamer was clogged, which meant that far too little water was reaching the boiler, and you can see the end result.
I installed a 70W ARS CP4 pump:
https://www.coffeemakers.de/Pumpe-Inv...
But the 65W CP3 fits better because of the space for the thermal fuse:
https://gastrotiger.at/shop/vibration...