the weired thing is, if I run 15.1R5 and load the factory-default configuration, the alarm goes aways after after rebooting the switch. It's only shown a few seconds after reboot and it's cleared/normalized by the switch itself.
Also the temperature of sensor FPC 0 GEPHY2 is shown under "show chassis environment"
Once I modify the configuration (nothing special, only interfaces, vlans, ntp) the sensor creates an alarm after rebooting the switch.