Just did some more troubleshooting...

Soo, new firmware. Now it shows: 364C extruder, 36C heatbed. The heatbed is a bit too high, but it responds to heat (goes upp to 43C when I hold the thermistor). When I plug the extruder thermistor into the heatbed socket, it shows 24C, and goes upp to 34C when I hold it. Now, to the odd part, the extruder always shows 364C, no matter if it is plugged in or not.