Solved by: The pre K value was set to 0.1 as described in the documentation. I set it back to 1. With one every measurement is working properly.
Question: Could you explain in more detail how the pre and post K and Q values are working. I'm a bit confused. Obviously its just a matter of scaling since for Pt1000 the pre K factor need to be set to 0.1. So for Pt500 the pre K factor should be set to 0.5? Am I correct?