Update:
It is a littele bit confusing because now the heatbed is working again....but in the wrong direction (when I turn it ON it stops heating).
Maybe I can fix this by editing pins.h in the firmware, i will try this later.
I read threads of other users who had similar problems.
However, if theres need to replace the MOSFET, I don't now where to get this spare.
Freiheit heisst das zu tun was man will, solange man niemand anderem schadet.