ical.net - issues with oulook and google agenda subscriptions - outlook

Question

I use a standard process to create a calendar with the library iCal.NET (see example below). The calendar is created (I can see it when I check the variable "output" - see below). But I face issues when I try to use my page to create a subscription, either in Google Agenda or in Outlook. In Outlook 365, I get the message "check your URL" ; In Google Agenda and with Outlook WebApp (I have seen that there were possibly identification issues with Internet calendars in Outlook), the calendar seems to work (I see the address in the agendas on the left), but the events are not imported.
The calendar is created via an aspx page with a querystring : something like https://mysite/mysubdirectory/mypage.aspx?ref=12345).
With Outlook, I have tried both webcal://mysite/mysubdirectory/mypage.aspx?ref=12345 and webcals://mysite/mysubdirectory/mypage.aspx?ref=12345
I am using Visual Studio 2022 and the latest version of ical.NET (4.2.0), my target is Framework 4.7,
What am I doing wrong ?
Dim dthor As DataTable = dstry.Tables.Add("trydate")
dthor.Columns.Add("ref", Type.GetType("System.Int32"))
dthor.Columns.Add("debut", Type.GetType("System.DateTime"))
dthor.Columns.Add("fin", Type.GetType("System.DateTime"))
dthor.Columns.Add("detail", Type.GetType("System.String"))
Dim drow As DataRow
drow = dthor.NewRow
drow("debut") = # 01/05/2022 08:00#
drow("fin") = # 01/05/2022 10:00#
drow("detail") = "Essai 1 éà"
drow("ref") = 1234
dthor.Rows.Add(drow)
drow = dthor.NewRow
drow("debut") = # 01/05/2022 11:00#
drow("fin") = # 01/05/2022 12:30#
drow("detail") = "Essai 2 èéà"
drow("ref") = 1235
dthor.Rows.Add(drow)
Dim iCal As New Ical.Net.Calendar()
iCal.AddTimeZone(New VTimeZone("Europe/Paris"))
iCal.ProductId = "my code"
iCal.Method = "PUBLISH"
iCal.Scale = "GREGORIAN"
For Each dr As DataRow In dthor.Rows
Dim evt As New CalendarEvent
With evt
.DtStart = New CalDateTime(CDate(dr("debut")))
.DtEnd = New CalDateTime(CDate(dr("fin")))
.Summary = dr("detail")
.Uid = dr("ref")
.DtStamp = New CalDateTime(DateTime.Now)
End With
iCal.Events.Add(evt)
Next
Dim serializer As New CalendarSerializer()
Dim output As String = serializer.SerializeToString(iCal)
Response.ContentType = "text/calendar"
Response.Write(output)
Response.End()
End Sub