We're unable to run a sales report per items that were used in an assembly, gerdgoebel.
This is because they were sold as groups and not individually. You'll want to get a list of all the items that comprise the group. Then, compare it with the quantity in the Sales by Item Detail report. This way, you would still know how much of the items were sold.
Go to the List menu and select Item list.
Look for the assembly item.
Right-click on it and select Print Assembly Item.
Check out these articles as well about running reports for additional reference: