After that I have to close the window with the x button on the right upper corner. :-(
When I create a new item in the standard way then the form is closed after saving and submitting. Any help is greatly appreciated...
window.open('Website-URL/Lists/PosNr/NewForm.aspx?Source=Website-URL/Lists/PosNr/AllItems.aspx&RootFolder=&isDlg=1','_blank', 'width = 920, height=1100, top=0, left=0')