I changed it to this:
[vba]Sub GemSom()
Dim ws As Worksheet
Set ws = Sheets("Kasserapport")
If Test(ws) = False Then
With Sheets("Kasserapport")
ActiveWorkbook.SaveAs CheckMakePath("G:\" & _
Sheets("Kasserapport").Range("H4").Text & "-huset" & "\" & "Beboere" & "\" & _
Sheets("Kasserapport").Range("D2").Text & "\" & "Regnskab" & "\" & _
Format(Sheets("Kasserapport").Range("A4"), "yyyy")) & _
"Regnskab " & Format(Sheets("Kasserapport").Range("A4"), "mm-yyyy") & _
" " & Sheets("Kasserapport").Range("D2").Text & ".xls"
End With
Else
MsgBox ("Udfyld f?rst Beboernavn og Startdato.")
End If
End Sub

Function Test(ws As Worksheet) As Boolean
If ws.Range("A4") = "" Or ws.Range("A4") = "01.01.00" Or ws.Range("D2") = "" Or ws.Range("D2") = "Beboernavn" Then Test = True
End Function[/vba]
in order to not allow file to be saved with dummy-data in D2 or A4.

Now it works.

Trapping on-close have to wait to another day.

Thank you. Tonight you are my hero