Thanks for the suggestions. Those are for sure valid points but I think I can exclude them both. At least during calibration process the Heli was not connected via Adapter to the Laptop and as the Geolink was mounted on the boom I needed some space to complete the acrobatic needed to complete the calibration. So I was at least one meter away from the laptop. And as said during the second tests (where it began to become better) I had the same placement and setup.
Yes it is a torque tube tail drive but during test and calibration the motor is always disconnected for security reasons and therefore also some magnetic influences through a driven tail can be excluded.