First Approach: I renewed thermal grease between chip and heat sink (heat sink is soldered to mainboard and has to be desoldered before) Result: after resoldering with new grease problem seemed to have even worsened. 50degC and shut-off was reached definitely faster - even without rear cover

Next attempt: I substituted heat sink by a bigger one (as in picture below later used fan): Heat sink was so big it hardly fit below rear cover; I temporarily fixed it with wire strap to mainboard and to press it against the chip

it worked with removed back cover for more than an hour with reaching about only 48 deg C. After applying rear cover (cover fortunately pressed the heat sink against chip) sensor again exceeded 50 deg C after about half an hour because of worse air circulation inside cover and device shut off .

As I needed a reliable solution I gave up with passive thermal cooling and tried active one