Fixen des Table Problems: Lösung war :

ALLE Rows und Cells innerhalb der ForEach-Schleife neu zzu erstellen sowie den ViewState-Mode der Tabelle auf False zu seten damit diese bei jedem Postback nicht neu geladen iwrd.

Customers/CustomsAviso.aspx

@@ -151,7 +151,7 @@
                                              <!-- <input class="form-control mb-4" id="tableSearch" type="text"
                                                 placeholder="Einen Begriff eingeben oder den Anfang davon ...">-->
                                              <div id="conovertab" style="overflow-x:auto;"margin-left="-2px"> 
-                                                   <asp:Table id="normaltable" runat="server" Width="100%">
+                                                   <asp:Table id="normaltable" runat="server" Width="100%" EnableViewState="false" >
                                                     <asp:TableHeaderRow ID="tbl_Header_row" runat="server">                                                        
                                                             <asp:TableHeaderCell ID="tbl_HeaderCellIndexNr" runat="server">Index-Nr</asp:TableHeaderCell>
                                                             <asp:TableHeaderCell ID="tbl_HeaderCell_ART" runat="server">ART</asp:TableHeaderCell>

Customers/CustomsAviso.aspx.vb

@@ -5,6 +5,18 @@ Imports System.Web.UI.HtmlControls
 Partial Class Kundenbereich_Default
     Inherits System.Web.UI.Page
 
+    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
+        tbl_Header_row.TableSection = TableRowSection.TableHeader
+
+        DataBind()
+
+        If (Page.IsPostBack And normaltable.EnableViewState = False) Then
+
+            normaltable.EnableViewState = True
+        End If
+    End Sub
+
+
 
     Protected Sub LogOutBtn1_Click(sender As Object, e As EventArgs)
 
@@ -30,17 +42,7 @@ Partial Class Kundenbereich_Default
         'MsgBox(Session.Item("PW").ToString)
         Kdnrtext = Session.Item("CustomerID").ToString
 
-        Dim tbl_CellIndex = New TableCell
+
-        Dim tbl_CellArt = New TableCell
-        Dim tbl_CellKundenNr = New TableCell
-        Dim tbl_CellStatus = New TableCell
-        Dim tbl_Cellvon = New TableCell
-        Dim tbl_Cellbis = New TableCell
-        Dim tbl_CellAbsender = New TableCell
-        Dim tbl_CellEmpfaenger = New TableCell
-        Dim tbl_CellLKWs = New TableCell
-        Dim tbl_CellKundAuftNr = New TableCell
-        Dim tbl_CellInfo = New TableCell
 
         dt = Nothing
 
@@ -109,11 +111,22 @@ Partial Class Kundenbereich_Default
             'sb.Append("</table>")
             ' ltTable.Text = sb.ToString
 
+
+            For Each d In dt.Rows
                 Dim Cell = New TableCell
+                Dim tbl_CellIndex = New TableCell
+                Dim tbl_CellArt = New TableCell
+                Dim tbl_CellKundenNr = New TableCell
+                Dim tbl_CellStatus = New TableCell
+                Dim tbl_Cellvon = New TableCell
+                Dim tbl_Cellbis = New TableCell
+                Dim tbl_CellAbsender = New TableCell
+                Dim tbl_CellEmpfaenger = New TableCell
+                Dim tbl_CellLKWs = New TableCell
+                Dim tbl_CellKundAuftNr = New TableCell
+                Dim tbl_CellInfo = New TableCell
                 'tr = New HtmlTableRow()
                 Dim tr = New TableRow
-            For Each d In dt.Rows
-
                 tbl_CellIndex.Text = i
                 tr.Cells.Add(tbl_CellIndex)
                 tbl_CellArt.Text = ""
@@ -155,9 +168,11 @@ Partial Class Kundenbereich_Default
 
                 i += 1
             Next
+
         End If
 
     End Sub
+
     Protected Sub rbt_Freig_Click(sender As Object, e As EventArgs)
         MsgBox("Es funzt!")
         ' datepicker1.Visible = True
@@ -187,9 +202,7 @@ Partial Class Kundenbereich_Default
 
     End Sub
 
-    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
+
-        tbl_Header_row.TableSection = TableRowSection.TableHeader
-    End Sub
 End Class