Success! The laptop boots again after clearing the BIOS settings.
Thanks to the hint by @lead4good: leave the battery connector disconnected for a while. There is no need to flash the BIOS.
Steps:
- Disconnect the laptop from the charger.
- Open the laptop.
- Lift the black tape that runs along the side of the battery. Lift it on the motherboard side were the connector is located.
- Gently push the battery connector (the one with red, white, blue and black wires) out of its socket.
- Wait a while (try one hour) and then reconnect the battery.
(I also pressed the power button for a few seconds while the battery was disconnected.)