Ist8310 external magnetometer not recognized

I have the following issue with my GPS-Compass module. The compass is a ist8310 type. However, no external magnetometer are found and if I try to start the driver over QGC (Command. start ist8310) it does not work.
Did I miss any parameter or other pitfalls?
I am using said GPS-Module (GPS, Switch LED and Buzzer work fine) and a Pixhawk 4 mini on the latest stable.
Thanks a lot!