Hi @Juraj,
Thanks for your quick answer.
I am using an Arduino MKR 1010 WiFi board (but I have replaced the original firmware with my Rust app). To test if the Nina chip works fine I reprogrammed (several times) the original bootloader to be able to test the board with Arduino IDE (I got the pinout trace from there, using Serial.print()).
I will check the post you suggested, but normally GPIO0 is used by my software as explained here:
