after a few days root cause analysis I found the reason:
At some heater cartridges the wire insulation is not firm, moving to far and revealing the bare wire.
If this wire touches the hotend, 24V voltage is applied to the hotend even when the extruder is not heating.
No if the thermistor breaks like the glass pearl is breaking and revealing the wires and those wires are touching the hotend, you will have the 24V voltage no on the thermistor line T0 wich is directly linked to the T0 pin of the ATMEGA2560 chip on the board. This most likely is killing the measuring capability of at least the T0 pin which will result in crazy temp reading on the display.
So better take precautions:
Every time your are doing something with the hotend it is recommended to check if the heater cartridge or the thermistor is touching the metal parts of the hotend.
REASON: If both the cartridge wire an the thermistor wire it touching the metal, you instantly fry the mainboard!
HOW TO CHECK:
Just unplug the cartridge and thermsitor plugs on the extension board, use a multimeter in diode mode and check the metal hotend vs. every line of the wiring.
Heater cartridge: The heater cartridge wires need to be bent 90 degree so make sure your heater cartridge is at least even or about 1mm outside the heatend. Best is if you also turn the cartridge vertically, so the little piece of wire exposed has enough clearance to the hotend. Advanced: you can also get yourself some hight temp silicone (+350°C e.g. K2 bond black silicone) or even exhaust assembly paste (+1000°C e.g. holts firegum but this will not be flexible) to seal the bare wires. this stuff might also help you with some hotend sealing...
Use a PTFE washer for the screw pushing the thermistor into the hotend. You might get the same result with a PTFE plumbing tape wrapper around the screw and the thermistor wires. Also possible is to use the high temp silicon, to just glue the thermistor into place, but this is somehow permanent.
Currently Geeetech is looking into the problem and trying to find an improvement and maybe a silicon sock will help in insulation the hotend.
Meanwhile I am waiting for some replacement ATMEGA2560 chisp trying to just resolder the new ones to the old board. If this is successful, you might have a cheaper solution than replacing the whole board.
I will keep you posted.
BTW: for further tips you can always check out: