I have a MDI application with 2 windows each containing a datagrid (implemented
<class name="WorkTimeRecording" table ="[WorkTime-Recording]" lazy="false">
<property name="IDEmployee" column="[ID-Employee]" />
The solution was, that my former colleague implemented the function with different levels of collecting data. The class WorkTimeRecording has different depths (basic, medium, complete - the reason is to get performance and just loading the data to the repository, which were needed) to fill the instance. When I switched to complete, the variable employee in WorkTimeRecording was filled correctly, no matter which other Windows were open.
So it's not a nhibernate issue as I thought, but loading enough data at the right time... and to understand the code, other people have written ;)