Option Explicit
Const MYRANGE As String = "D5:F5"
Private Sub Workbook_SheetBeforeDoubleClick(ByVal Sh As Object, ByVal Target As Range, Cancel As Boolean)
If Sh.name <> "Tabelle" Then Exit Sub
If Target.Cells.Count > 1 Then Exit Sub
If Intersect(Target, Sh.Range(MYRANGE)) Is Nothing Then Exit Sub
Cancel = True
If Target.Value = "" Then
Target.Value = "x"
Else
Target.Value = ""
End If
End Sub