Serial characters are corrupted I try to send an "a" string via serial with

Serial.println("a");


and I receive "Oy=" in Ascii or {4F}{79}{3D} in Hexa.

Where should I start investigating? What could be wrong?  Baudrate is 9600 on all sides.

PS: I suspect the cable. Any hints how to check it?
On your setup function

Serial.begin(115200);//&lt;- 115200 is the baud rate, serial monitor must match this speed
while(!Serial);//&lt;-- wait for Serial to be ready


see: https://electronics.stackexchange.com/questions/9264/what-standard-uart-rates-are-there for more baud rates