Hello @Berni4
the C3' TX (GPIO21) is connected to the USB UART RX pin; in other words the C3 is driving the line, therefore your code can blink an LED on GPIO21 just fine.
However the C3' RX (GPIO20) is connected to the USB UART TX pin; in this case the USB UART and the C3 are both trying to drive the line and it looks like the USB UART is winning.
In short, without hardware modification I don't see a way to use GPIO20 for anything else than UART functionality.
Thanks
Felix