Surpresso S40 - Fault 1

Hello,

I have the following problem with my Siemens surpresso S40:
After switching on I can get coffee or milk foam or steam normally. I have only noticed that the machine occasionally shows the device is heating. Then the machine sometimes stands switched on for 2-3 hours in the operational state.
At some point fault 1 appears and then alternately very briefly again the device heats and so on. I also suspect that the machine then heats up a little bit again, because the sheet metal on top gets relatively hot. As soon as this malfunction occurred, I completely switched off the machine (standby), waited about 20-30 minutes and then switched it on again. Now it works again without any problems.
I measured the fuse, so it has 0 ohms full passage. I also bought the thermosensor new and it shouldn't be because of that.
may anyone have a hunch what I can still rule out?

I thank you guys...