problem solved by shutdown/start (not just by restart!)
An update to my problem. Yesterday, I decided to turn the MBP completely off. To my great surprise, it helped! I'm genuinely surprised, because a day before, I tried to restart the macbook and it did not help. Now, after the complete shutdown and new start, the macbook keeps it's previous temperature level 50˚C +/-5˚. I will definitely watch it more closely from now, to try identify the starting sequence. The problem is, that there is nothing seen in the Activity Monitor that would point to the source of problem.