When zoom is applied in IE browser and button coordinates are not an integer RadImageButton tries to convert them to an integer on the server on postback and fails.

Workaround:

			<telerik:RadImageButton runat="server" ID="rib1" Width="50" Height="50" OnClick="rib1_Click" Image-Url="~/images/myImage.png"></telerik:RadImageButton>
			<script>
				Telerik.Web.UI.Button.ImageCoordsFunctionality.prototype._calculateCoords = function (e) {
					var container = this._ui.element;
					var pos = $telerik.getLocation(container);
					var scroll = $telerik.getScrollOffset(container, true);

					return new Telerik.Web.UI.Point(parseInt(e.clientX + scroll.x - pos.x), parseInt(e.clientY - pos.y + scroll.y));
				}
			</script>