Thanks for your comments.
How this could happen [ignazw] ? It’s not a temperature problem, I live in Switzerland, and we get a really fresh summer. It’s not an overcurrent problem, I don’t use my phone to play games, neither watch video nor intensive applications.
As suggested by Lidwien, my phone is usually connected to the charge (slow charge mode), the goal is minimizing the number of cycles par week (around 2 to 3). It was a good idea for the FP1: the battery lasted more than 4 years (until no longer for sale), then my wife and my son had already changed their FP1 battery.
Then I could measure the voltage, to check an overvoltage because of the permanent charge. It’s theoretically not possible with a smart charger.
So I’m going to contact Fairphone support quickly to change my battery before calling the firemen.