Thank you for your reply.
I already known (more than 50 times) the strange attitude of the IDE once trying to upload a sketch including a Serial.read command.
The typical process is an endless compilation with freeze of the IDE, ending only when pressing the reset button (sign of a complete compilation, but a stucking upload).
The second uploading trial must be proceeded after a quick double reset button press, in order to switch to a deep reset. Doing so changes the serial port, and after the change in the IDE, the upload process runs til the end.
But... the sketch does not run, displaying a column of strange characters on the serial monitor.
I can admit that this device is not much used, but it is strange not finding any similar problems on the net...