Hi Erdin, thanks for your answer.
Yes, voltage at 5V pin is 4.7-4.8V so seems ok, so the voltage regulator should be fine right? Also the 3.3 pin is ok and Vin pin measures the 11.0V I'm puting at the jack connector from external power as well.
Anyway the power through USB was always fine before, but it's obvious that something is wrong if I can read Serial data unplugging the 5V connector of the USB-serial adapter and weird symbols appear at the serial monitor.
About the temperature, I will try to measure it and confirm if it's too hot.
Will let you know, thanks!