Serial Monitor displays only blocks (SOLVED)

[J-M-L]

@phvg

 int PB0 = 16; // D0 = LED at USB connector (blink to show heartbeat)
int PBH = 1;  // TX = LED on PCB for heartbeat
int PB8 = 2;  // D4 = Push Button 8 = grey LED at antenna (only after boot)

would those conflict with the Serial UART ?
which exact board do you have?