Problem Fixed !!
This error generally occurs when a hidden controls fall under AJAX. Suppose you have a textbox on the page and it gets hidden and displayed based on some setting and the same textbox is added under ajax setting on the .aspx page with some submit button. Now if the textbox is hidden based on setting and we hit submit button then it will generate "Sys.InvalidOperationException: Could not find UpdatePanel with ID " error.
The best practice in such cases, when the controls are displayed based on some setting is to Ajaxify controls pro-grammatically. Add the controls on the page_load method and also with events of control like radcombobox selectedIndexChange event if some dropdowns are dependent.
radAjaxManager.AjaxSettings.AddAjaxSetting(cmdSubmit, txtAddress, AjaxLoadingPanel1, UpdatePanelRenderMode.Inline)