Refrigerator cools beyond freezing, thermostat?

Hi all,

My Samsung RB31FERNDSA (Dutch market model) is overcooling the refrigerator to the point where all liquids become frozen. We tried defrosting it for 24 hours but as soon as power's back the same issue occurs. I understand that there is a valve which opens as soon as the temp is too high and shuts when it reaches set value. I think that valve may be stuck in the open position, or otherwise the internal thermostat is busted. Does anyone here know how to reach either one? I think they are located behind the back cover inside the fridge, but am clueless how to open that up (without breaking it)

Thanks so much in advance