I confirm that this has solved my issue and mark it on the title for the posterity, many thanks!
edit: apparently titles cannot be edited after a while, if a moderator could add "[SOLVED]" to this thread's title, it would be helpful for others 🙂
@fabryz OK, I gave it more thought and your problem may be that you have a program construct like "while true Serial.print(somevariable)" that needs to serial print and receive a confirmation.
Please post your program for further assistance.
-Terry