Temperature shows wrong Value

Hello and good morning,

I am a newbie and play with K8200 now for a few weeks collecting experience. In general I am amazed with this nice product!

After a period of well printed objects I discover a problem with the bed-temperature that is shown in repetier host:

at 19° it shows 31.4° (extruder temp shows correct 18.8°)
when disconnected it shows 24°, when short-circuited it shows 761°

I switched the cables of both NTCs at the board but still the same effect. The extruder temp works as expected, but the bed-temp brings false values.

I measured the NTC (at the end of the wire) and it shows ~110K at 19° so I suppose that is not the problem.

Has anyone an idea what to do?
btw: where do I find instructions to upgrade Marlin? I looked at velleman h/p and K8200.eu but with no success.

Thanks in advance, Mike

Doesnt anyone know where to find the instruction how to install the firmware on the board? I read in another forum that 2 pins need to be connected/soldered.

Please can a mod from Velleman help?

Thanks in advance, Mike

Please read this part of manual :