Se avete problemi modificate il codice come segue.

Nel Calendar.aspx :

<script runat="server">

Sub ShowDate(ByVal sender As Object, ByVal e As EventArgs)
Response.Write("<S" + "CRIPT language=""Javascript""><!--" & vbCrLf)
Response.Write("window.opener.document.forms(0)." & Request.Params ("campo") & ".value='")
Response.Write(cal.SelectedDate.ToString("dd/MM/yyyy") & "';" & vbCrLf)
Response.Write("self.close();" & vbCrLf)
Response.Write("--></SCRIP")
Response.Write("T>")
End Sub

</script>

Nella pagina chiamante (Quella del link) :

a href="javascript:popdate('../calendar.aspx','data', '');">...</a


Quindi sostituite in calendar.aspx la riga
Response.Write("window.opener." & Request.Params("campo") & ".value='")

con
Response.Write("window.opener.document.forms(0)." & Request.Params("campo") & ".value='")

Nella pagina chiamante invece sostituite
href="javascript:popdate('../calendar.aspx','frm.data', '');"

con
href="javascript:popdate('../calendar.aspx','data', '');"