Siemens eq6 cold drinks

Hi allHave
an eq6 300 with supposedly original 220 cups on the clock bought at auction.
Machine however only brings cold drinks. After 2 or 3 pulls it gets minimally warmer at times and then colder again, but definitely not coffee temperature.
Does anyone have any idea what this could be.
There is supposedly no invoice for warranty purposes according to the seller.
Many greetings

If it hasn't happened yet, simply decalcify properly, e.g. with https://www.amazon.de/ceragol-ultra-Premium-Entkalker-500ml/dp/B000FZ4I6W/ref=sr_1_2?__mk_de_DE=%C3%85M%C3%85%C5%BD%C3%95%C3%91&dchild=1&keywords=ceragol&qid=1616575272&sr=8-2.

If this does not help, the instantaneous water heater may be over.