• OK guys that's not a problem related to code...
    Page 12 of HTU21D datasheet (the one from TE, not an equivalent from Silabs or Sensirion), it is written under "Diagnostic status" that if zeros comes out of the conversion buffer, it means that the sensor is in "open circuit" condition. In other terms, my IC is broken :-/
    I just tried with another IC and this works perfectly. Sorry about time you guys spent helping me. I did not consider it could be an IC problem, as I was able to retrieve the temperature easily.