This notebook is a Lenovo Thinkpad W520 with a built-in X-Rite calibrator, I just used the Color Calibrator software that came with the notebook.
Just to check why the problem occured I also used a Squid 3 colorimeter with the Basiccolor calibration software - and now it works. It seems the built-in device is the problem.
So, now it works, thanks for your help.