Alright... i finally "fixed" it.
Everything was set to baud rate 115200 (x-IMU, both serials and serial monitor) and, as i mentioned in the post before, the serial monitor got stuck.
Now i have set the baud rate of "Serial" and the serial monitor to 9600 and it works.
...
//------------------------------------------------------------------------------
// Functions
void setup() {
Serial.begin(9600); // for sending data to computer
Serial1.begin(115200); // for receiving data from x-IMU
}
...
I asked the developer of the x-IMU if this is just accidentally or if this should work that way, whereas he said "I am not sure why this problem affects your system but not mine."
Best, milla