CPU core temps quickly at ~100°C with no load

This problem started a few month ago where this occurred a few times randomly but could be fixed with 1-3 reboots. Now this is a permanent problem:
When my PC boots it takes about 5sec until my watercooling fans start to tu at 100% RPM, about 10sec later when the desktop appeared and I instantly start SpeedFan it tells me that core temps are already at 80-90°, half a minute later they are all on 98-100°C. But even after a few minutes at this temperature (didn't want to try it longer) I do not get a force shut down, neither is the air temperature at the fans any higher than room temp. So is it possible that the sensors are broken or is it more likely that the cooler is not mounted properly anymore (even if it looks like it).

I have a Intel Xeon 1230v3 and a Corsair Hydro Series H75 cooling.

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