Hello,
I am using UltraComboEditor to allow a user to select multiple values for a single grid row.
That's how I init the UltraComboEditor control on from load:

ddParameters.CheckedListSettings.EditorValueSource = Infragistics.Win.EditorWithComboValueSource.CheckedItems;
ddParameters.CheckedListSettings.ItemCheckArea = Infragistics.Win.ItemCheckArea.Item;
ddParameters.CheckedListSettings.ListSeparator = ";";
ddParameters.CheckedListSettings.CheckBoxStyle = Infragistics.Win.CheckStyle.CheckBox;

And use this code on grid's InitiLayout:

e.Layout.Bands[0].Columns["ViewRequiredParamIds"].EditorComponent  =  this.ddParameters;
e.Layout.Bands[0].Columns["ViewRequiredParamIds"].Style = Infragistics.Win.UltraWinGrid.ColumnStyle.DropDownValidate;

Any time I check more than one value in dropdown, I get this error message:

Unable to update the data value: Value in the editor is not valid.