Dim wb As Microsoft.Office.Tools.Excel.Workbook
Dim xlapp As New Microsoft.Office.Interop.Excel.ApplicationClass
wb = xlapp.Workbooks.Open(Application.StartupPath & "\Rechnung.xls", True, _
  False)  <---hier stoppt die Anwendung mit der Fehlermeldung
 
Dim ws As Microsoft.Office.Tools.Excel.Worksheet = xlapp.ActiveSheet
Dim s As String = ws.Range("A1").Value
ws.Range("A1").Value = "zzz"
wb.Save()
wb.Close()
xlapp.Quit()

VB@rchiv its the Best