Hi,

I tested sample where grid has no rows, has addNewButton and editor with dd/MM/yyyy. It worked ok and picked up value of date, persisted it, could edit cell after a postback after after editing new row or cell in existing row.

If grid has custom editor, then there is no need to set Format of column.
Maybe after a postback something happens to grid settings: column editor is lost or cell value is in wrong format. Try to experiment with simplest possible application (grid and editor on form and only initialization codes for grid and its editor) and test if it works correctly.

I had codes as below.

aspx:

<igtxt:WebDateTimeEdit ID="WebDateTimeEdit1" runat="server" EditModeFormat="dd/MM/yyyy">
</igtxt:WebDateTimeEdit>
<igtbl:UltraWebGrid ID="UltraWebGrid1" runat="server" OnInitializeLayout="UltraWebGrid1_InitializeLayout" ... >
       <displaylayout ... AllowAddNewDefault="Yes">
    ...

aspx.cs:

protected override void OnInit(EventArgs e)
{
  base.OnInit(e);
  this.UltraWebGrid1.InitializeDataSource += new Infragistics.WebUI.UltraWebGrid.InitializeDataSourceEventHandler(UltraWebGrid1_InitializeDataSource);
}

void UltraWebGrid1_InitializeDataSource(object sender, Infragistics.WebUI.UltraWebGrid.UltraGridEventArgs e)
{
  DataTable dt = new DataTable("Customer");
  dt.Columns.Add("CustomerID", typeof(int));
  dt.Columns.Add("CustomerName", typeof(string));
  dt.Columns.Add("Address", typeof(string));
  dt.Columns.Add("Date", typeof(DateTime));
  this.UltraWebGrid1.DataSource = dt;
}
protected void UltraWebGrid1_InitializeLayout(object sender, Infragistics.WebUI.UltraWebGrid.LayoutEventArgs e)
{
  this.UltraWebGrid1.Columns[3].EditorControlID = this.WebDateTimeEdit1.UniqueID;
  this.UltraWebGrid1.Columns[3].Type = Infragistics.WebUI.UltraWebGrid.ColumnType.Custom;
}