just figured out - after an extensive search- where the MINTEMP ERROR reading on my FT5 is coming from .
It turns out that the cable from the thermistor to the motherboard is broken at a specific place about half way in the cable chain. If I bring the extruder to Y-position 0, everything is OK but if I jog the carriage in Y-direction using Pronterface, suddenly temperature from the thermistor drops to minus 15°C. It took me quite a while to figure this out so I want to spare all of you the hassle of searching if you encounter the same problem : check in Pronterface the reading of the thermistor while moving the carriage in X- and Y-direction the entire stroke length and see what happens . If it suddenly changes, the cable is broken in the cable chain.