after posting the question above I found the solution to my problem.
The problem based on a wrong multiplicity-value.
I used 'one' although I needed 'Many'.
In my application an Instance of X is related to an instance of Y. So I though X would have always exactly one instance assigned. Thereby I forgot, that a instance of Y may be assigned to several instances of X.
Because of Transaction security the system claimed an exception, because of an multiple 'assignment '.
Thanks to all thathas thaught about a solution to my problem.