There should not be any difference internally between Upper and Lower case. This sounds like a bug to me. If you are able to duplicate this in a small sample project, you should Submit an incident to Infragistics Developer Support so they can check it out.

Click on "Move next" twice and you'll see the exception pop up...

<Form1.cs>

using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            SetupControls();

            // ===================================================================================
           
            List<Data> x = new List<Data>() {
                new Data("Cat"),
                new Data("Rat"),
                new Data(null),
                null,
                new Data(""),
                new Data("Fat") };

            BindingSource dataBindingSource = new BindingSource();
            dataBindingSource.DataSource = x;

            bindingNavigator1.BindingSource = dataBindingSource;

            BindingSource dataBindingSource2 = new BindingSource();
            dataBindingSource2.DataSource = dataBindingSource;
            dataBindingSource2.DataMember = "Insides";
            ultraGrid1.DataSource = dataBindingSource2;
           
            ultraTextEditor1.CharacterCasing = CharacterCasing.Lower;
            ultraTextEditor1.DataBindings.Add("Text", dataBindingSource2, "Name");

            ultraGrid1.Select(); // give focus
        }
    }

    public class Data
    {
        public class Inside
        {
            private string _name;
            private Inside() { }
            public Inside(string x) { _name = x; }
            public string Name { get { return _name; } }
        }

        public List<Inside> _Insides = new List<Inside>();
        public List<Inside> Insides { get { return _Insides; } }

        private Data() { }
        public Data(string x) { if (x != null) { _Insides.Add(new Inside(x)); _Insides.Add(new Inside(x + " 2")); } }
    }








    public partial class Form1
    {
        private Infragistics.Win.UltraWinEditors.UltraTextEditor ultraTextEditor1 = new Infragistics.Win.UltraWinEditors.UltraTextEditor();
        private System.Windows.Forms.BindingNavigator bindingNavigator1 = new BindingNavigator();
        private System.Windows.Forms.ToolStripButton bindingNavigatorAddNewItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripLabel bindingNavigatorCountItem = new ToolStripLabel();
        private System.Windows.Forms.ToolStripButton bindingNavigatorDeleteItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripButton bindingNavigatorMoveFirstItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripButton bindingNavigatorMovePreviousItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripSeparator bindingNavigatorSeparator = new ToolStripSeparator();
        private System.Windows.Forms.ToolStripTextBox bindingNavigatorPositionItem = new ToolStripTextBox();
        private System.Windows.Forms.ToolStripSeparator bindingNavigatorSeparator1 = new ToolStripSeparator();
        private System.Windows.Forms.ToolStripButton bindingNavigatorMoveNextItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripButton bindingNavigatorMoveLastItem = new ToolStripButton();
        private System.Windows.Forms.ToolStripSeparator bindingNavigatorSeparator2 = new ToolStripSeparator();

        void SetupControls()
        {
            // this
            this.Size = new Size(400, 300);

            //
            // ultraTextEditor1
            //
            this.ultraTextEditor1.Location = new System.Drawing.Point(271, 39);
            this.ultraTextEditor1.Name = "ultraTextEditor1";
            this.ultraTextEditor1.Size = new System.Drawing.Size(100, 21);
            this.ultraTextEditor1.TabIndex = 1;
            this.ultraTextEditor1.Text = "ultratexteditor1";
            this.Controls.Add(ultraTextEditor1);

            SetupBindingNavigator();
            SetupGrid();
        }

        private Infragistics.Win.UltraWinGrid.UltraGrid ultraGrid1 = new Infragistics.Win.UltraWinGrid.UltraGrid();

        private void SetupGrid()
        {
            //
            // ultraGrid1
            //
            this.ultraGrid1.DisplayLayout.BorderStyle = Infragistics.Win.UIElementBorderStyle.Solid;
            this.ultraGrid1.DisplayLayout.CaptionVisible = Infragistics.Win.DefaultableBoolean.False;
            this.ultraGrid1.DisplayLayout.MaxColScrollRegions = 1;
            this.ultraGrid1.DisplayLayout.MaxRowScrollRegions = 1;
            this.ultraGrid1.DisplayLayout.Override.BorderStyleCell = Infragistics.Win.UIElementBorderStyle.Dotted;
            this.ultraGrid1.DisplayLayout.Override.BorderStyleRow = Infragistics.Win.UIElementBorderStyle.Dotted;
            this.ultraGrid1.DisplayLayout.Override.CellClickAction = Infragistics.Win.UltraWinGrid.CellClickAction.EditAndSelectText;
            this.ultraGrid1.DisplayLayout.Override.CellPadding = 0;
            this.ultraGrid1.DisplayLayout.Override.HeaderClickAction = Infragistics.Win.UltraWinGrid.HeaderClickAction.SortMulti;
            this.ultraGrid1.DisplayLayout.Override.HeaderStyle = Infragistics.Win.HeaderStyle.WindowsXPCommand;
            this.ultraGrid1.DisplayLayout.Override.RowSelectors = Infragistics.Win.DefaultableBoolean.False;
            this.ultraGrid1.DisplayLayout.ScrollBounds = Infragistics.Win.UltraWinGrid.ScrollBounds.ScrollToFill;
            this.ultraGrid1.DisplayLayout.ScrollStyle = Infragistics.Win.UltraWinGrid.ScrollStyle.Immediate;
            this.ultraGrid1.DisplayLayout.ViewStyleBand = Infragistics.Win.UltraWinGrid.ViewStyleBand.OutlookGroupBy;
            this.ultraGrid1.Location = new System.Drawing.Point(13, 39);
            this.ultraGrid1.Name = "ultraGrid1";
            this.ultraGrid1.Size = new System.Drawing.Size(240, 187);
            this.ultraGrid1.TabIndex = 4;
            this.ultraGrid1.Text = "ultraGrid1";
            this.Controls.Add(ultraGrid1);
        }

        private void SetupBindingNavigator()
        {
            //
            // bindingNavigator1
            //
            this.bindingNavigator1.AddNewItem = this.bindingNavigatorAddNewItem;
            this.bindingNavigator1.CountItem = this.bindingNavigatorCountItem;
            this.bindingNavigator1.DeleteItem = this.bindingNavigatorDeleteItem;
            this.bindingNavigator1.Items.AddRange(new System.Windows.Forms.ToolStripItem[] {
            this.bindingNavigatorMoveFirstItem,
            this.bindingNavigatorMovePreviousItem,
            this.bindingNavigatorSeparator,
            this.bindingNavigatorPositionItem,
            this.bindingNavigatorCountItem,
            this.bindingNavigatorSeparator1,
            this.bindingNavigatorMoveNextItem,
            this.bindingNavigatorMoveLastItem,
            this.bindingNavigatorSeparator2,
            this.bindingNavigatorAddNewItem,
            this.bindingNavigatorDeleteItem});
            this.bindingNavigator1.Location = new System.Drawing.Point(0, 0);
            this.bindingNavigator1.MoveFirstItem = this.bindingNavigatorMoveFirstItem;
            this.bindingNavigator1.MoveLastItem = this.bindingNavigatorMoveLastItem;
            this.bindingNavigator1.MoveNextItem = this.bindingNavigatorMoveNextItem;
            this.bindingNavigator1.MovePreviousItem = this.bindingNavigatorMovePreviousItem;
            this.bindingNavigator1.Name = "bindingNavigator1";
            this.bindingNavigator1.PositionItem = this.bindingNavigatorPositionItem;
            this.bindingNavigator1.Size = new System.Drawing.Size(382, 25);
            this.bindingNavigator1.TabIndex = 3;
            this.bindingNavigator1.Text = "bindingNavigator1";
            this.Controls.Add(bindingNavigator1);
            //
            // bindingNavigatorAddNewItem
            //
            //this.bindingNavigatorAddNewItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            ////this.bindingNavigatorAddNewItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorAddNewItem.Image")));
            this.bindingNavigatorAddNewItem.Name = "bindingNavigatorAddNewItem";
            this.bindingNavigatorAddNewItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorAddNewItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorAddNewItem.Text = "Add new";
            //
            // bindingNavigatorCountItem
            //
            this.bindingNavigatorCountItem.Name = "bindingNavigatorCountItem";
            this.bindingNavigatorCountItem.Size = new System.Drawing.Size(36, 22);
            this.bindingNavigatorCountItem.Text = "of {0}";
            this.bindingNavigatorCountItem.ToolTipText = "Total number of items";
            //
            // bindingNavigatorDeleteItem
            //
            //this.bindingNavigatorDeleteItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            //this.bindingNavigatorDeleteItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorDeleteItem.Image")));
            this.bindingNavigatorDeleteItem.Name = "bindingNavigatorDeleteItem";
            this.bindingNavigatorDeleteItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorDeleteItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorDeleteItem.Text = "Delete";
            //
            // bindingNavigatorMoveFirstItem
            //
            //this.bindingNavigatorMoveFirstItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            //this.bindingNavigatorMoveFirstItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorMoveFirstItem.Image")));
            this.bindingNavigatorMoveFirstItem.Name = "bindingNavigatorMoveFirstItem";
            this.bindingNavigatorMoveFirstItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorMoveFirstItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorMoveFirstItem.Text = "Move first";
            //
            // bindingNavigatorMovePreviousItem
            //
            //this.bindingNavigatorMovePreviousItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            //this.bindingNavigatorMovePreviousItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorMovePreviousItem.Image")));
            this.bindingNavigatorMovePreviousItem.Name = "bindingNavigatorMovePreviousItem";
            this.bindingNavigatorMovePreviousItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorMovePreviousItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorMovePreviousItem.Text = "Move previous";
            //
            // bindingNavigatorSeparator
            //
            this.bindingNavigatorSeparator.Name = "bindingNavigatorSeparator";
            this.bindingNavigatorSeparator.Size = new System.Drawing.Size(6, 25);
            //
            // bindingNavigatorPositionItem
            //
            this.bindingNavigatorPositionItem.AccessibleName = "Position";
            this.bindingNavigatorPositionItem.AutoSize = false;
            this.bindingNavigatorPositionItem.Name = "bindingNavigatorPositionItem";
            this.bindingNavigatorPositionItem.Size = new System.Drawing.Size(50, 21);
            this.bindingNavigatorPositionItem.Text = "0";
            this.bindingNavigatorPositionItem.ToolTipText = "Current position";
            //
            // bindingNavigatorSeparator1
            //
            this.bindingNavigatorSeparator1.Name = "bindingNavigatorSeparator1";
            this.bindingNavigatorSeparator1.Size = new System.Drawing.Size(6, 25);
            //
            // bindingNavigatorMoveNextItem
            //
            //this.bindingNavigatorMoveNextItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            //this.bindingNavigatorMoveNextItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorMoveNextItem.Image")));
            this.bindingNavigatorMoveNextItem.Name = "bindingNavigatorMoveNextItem";
            this.bindingNavigatorMoveNextItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorMoveNextItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorMoveNextItem.Text = "Move next";
            //
            // bindingNavigatorMoveLastItem
            //
            //this.bindingNavigatorMoveLastItem.DisplayStyle = System.Windows.Forms.ToolStripItemDisplayStyle.Image;
            //this.bindingNavigatorMoveLastItem.Image = ((System.Drawing.Image)(resources.GetObject("bindingNavigatorMoveLastItem.Image")));
            this.bindingNavigatorMoveLastItem.Name = "bindingNavigatorMoveLastItem";
            this.bindingNavigatorMoveLastItem.RightToLeftAutoMirrorImage = true;
            this.bindingNavigatorMoveLastItem.Size = new System.Drawing.Size(23, 22);
            this.bindingNavigatorMoveLastItem.Text = "Move last";
            //
            // bindingNavigatorSeparator2
            //
            this.bindingNavigatorSeparator2.Name = "bindingNavigatorSeparator2";
            this.bindingNavigatorSeparator2.Size = new System.Drawing.Size(6, 25);
        }
    }
}