Ok, finally, it runs:

Do
With Application.Workbooks("Auswerter.xls").Worksheets( "data")
Set trng = .Range(.Cells(targetcounter, 1), .Cells(targetcounter,
6))
End With

With Application.Workbooks(filename).Worksheets("test2" )
Set rng = .Range(.Cells(counter, 1), .Cells(counter, 6))
rng.Copy Destination:=trng
End With

Still, if anyone could explain to me, why this works and not the other
way, it would be great.
Thanks again!
Steve