It looks like my thermometer locked on 302°C and altimeter at -22.999m were caused by a non-Garmin map installed on the watch which caused a problem with the software.
This is something to keep in mind for all those who use non-Garmin maps (and it took them 2 weeks to find out. The expected delivery times are 1 more week so in total I've lost the watch for 3 weeks).
Please if you find your barometer issue is solved by uninstalling those maps let me know.
Also, please let me know how and where to find such maps because I never installed one.
As a side note, even if one such map had been loaded, after performing the master reset (which is a standard procedure BEFORE RMA), that map couldn't possibly have been in use. So we have a map which hasn't been installed and in any case not in use, causing a software failure and this is Garmin Italy tech support claiming so.
(unbelievable... what's going on is really unbelievable. These guys really can't tell the sun from the m00n and what's worse is that they are completely unaware of WHO they're lying to)