JSON formatting error

Hello to all of you, I using JQuery Data Table to display data but i getting JSON formatting error when post data back from server to client.

JSP

<script type="text/javascript" charset="utf-8">

$(document).ready(function()
{
	$("#ticketList").dataTable({
		
		"bProcessing" : true,						// Enable processing indicator
		"bServerSide" : true,						// Configure data option - server or client 
													// Send XHR request to server
		"sServerMethod": "POST",						// GET or POST
		"bSort" : true,								// Enable sorting
		"bFilter" : false,
		"bAutoWidth" : true,			
		"sPaginationType" : "full_numbers",			// Paginate Type -> two_button (Previous && Next)
													// 				 -> full_numbers (First, Previous, 1, 2, Next, Last)		
		"bLengthChange" : true, 
		"iDisplayLength" : 10, 
		"sAjaxSource" : "${context}/app.view?tn=Ticket.SearchTicket", 
		// Override $.getJson() get data from server
		/*"fnServerData": function(sSource, aoData, fnCallback) {
            $.ajax({ "dataType": 'json',
                "type": "GET",			// GET or POST to server for getting data
                "url": sSource,
                "data": aoData,
                "success": fnCallback,
                "error": function(jqXHR, textStatus, errorThrown) {
                    console.log(txtStatus, errorThrown); 	// use alert() if you prefer
                }
            });
		 },  */ 

		"aoColumns": [
		      			//mData mDataProp
                      	{"mData": "id"}, 
                      	{"mData": "category"}, 
                      	{"mDataP": "status"}, 
                        {"mData": "assignee"}, 
                        {"mData": "urgency"}, 
                        {"mData": "action"}
                     ]
	});
});
</script>
<div class="clear" id="content">		
	<div class="box">
		<em>Search Ticket </em>	
		<div class="data">
			<div class="fb clear">
				<form id="searchTicketForm" name="searchTicketForm" action="${context}/app.view?tn=Ticket.SearchTicket" method="POST">
					<fieldset>
						<div class="field"> 
							<label>Ticket Id:</label>
							<input type="text" id="id" name="id"></input>
						</div>
						
						<div class="field"> 
							<label>Category:</label>
							<select id="category" name="category">
								<option value="account">Account</option>
								<option value="invoice" selected="selected">Invoice</option>
								<option value="rates">Rates</option>
								<option value="">No category</option>
							</select>
						</div>

						<div class="field"> 
							<label>Urgency:</label>
							<select id="urgency" name="urgency">
								<option value="high">High</option>
								<option value="medium" selected="selected">Medium</option>
								<option value="low">Low</option>
								<option value="">No urgency</option>
							</select>
						</div>
						
						<div class="field"> 
							<label>Status:</label>
							<select id="status" name="status">
								<option value="unassigned" selected="selected">Unassigned</option>
								<option value="in_progress">In Progress</option>
								<option value="solved">Solved</option>
								<option value="">No Status</option>
							</select>
						</div>
						<div class="clear"></div>
						<div id="error" class="field"></div>
						<br />
						<div class="field">
							<button class="dark" id="searchBtn" type="submit" name="search">Search Ticket</button>
						</div>
					  </fieldset>
				</form>
			</div>
		</div>
		<div class="data">
			<div id="ticketList_wrapper" class="dataTables_wrapper">
				<c:if test="${not empty tableView}">
					<div id="ticketList_length" class="dataTables_wrapper">
						<!--  Show
						<select name="ticketList_length">
							<option value="2">2</option>
							<option value="4">4</option>
							<option value="6">6</option>
						</select>
						entries 
						-->
					</div>
					<!-- <form name="searchUserForm" action="${context}/app.view?tn=Ticket.SearchTicket" method="POST"> -->
						<table id="ticketList" class="display" cellspacing="0" cellpadding="0" border="0">
							<thead>
								<tr>
									<th class="sorting"><b>Ticket ID</b></th>
									<th class="sorting_disabled"><b>Category</b></th>
									<th class="sorting_disabled"><b>Urgency</b></th>
									<th class="sorting_disabled"><b>Status</b></th>
									<th class="sorting_disabled"><b>Assignee</b></th>
									<th class="sorting_disabled"><b>Action</b></th>
								</tr>		
							</thead>
							<tbody>
								<tr>
									<td colspan="5" class="dataTables_empty" align="center">Loading data from server</td>
								</tr>
							</tbody>
						</table>
					<!-- </form> -->
				</c:if>
			</div>
		</div>
	</div>
</div>		

Servlet

@Override
	public TaskResponse execute(HttpServletRequest request,
			HttpServletResponse response) throws WebTaskException {

		String sEcho = null;
		String sSearch = null;

		HttpSession session = request.getSession();
		
		if (session == null) {
			throw new WebTaskException("Unauthorized Access");
		}

		sEcho = request.getParameter("sEcho");
		sSearch = request.getParameter("sSearch");
		// Get page number -> Pagination -> XHR request
		if (sEcho != null) {
			Integer id = null;
			String categoryStr = request.getParameter("category");
			String urgencyStr = request.getParameter("urgency");
			String statusStr = request.getParameter("status");
			int fetchRowSize = 100;
			int currentRow = 0;
			
			if (request.getParameter("iDisplayLength") != null) {
				fetchRowSize = Integer.parseInt(request.getParameter("iDisplayLength"));
				logger.log(Level.INFO, "Fetch Row Size " + fetchRowSize);
			}
			
			if (request.getParameter("iDisplayStart") != null) {
				currentRow = Integer.parseInt(request.getParameter("iDisplayStart"));
				logger.log(Level.INFO, "Current Row " + currentRow);
			}
			
			Ticket ticket = new Ticket();
			List<Ticket> ticketList = new ArrayList<Ticket>();
			
			if (request.getParameter("id") != null && request.getParameter("id") != "") {
				id = Integer.parseInt(request.getParameter("id"));
				ticket.setId(id.intValue());
			} else {
				ticket.setId(-1);
			}

			if (this.category.get(categoryStr) != null) {
				ticket.setCategory(this.category.get(categoryStr).intValue());
			} else {
				ticket.setCategory(-1);
			}
			
			if (this.urgency.get(urgencyStr) != null) {
				ticket.setUrgency(this.urgency.get(urgencyStr).intValue());
			} else {
				ticket.setUrgency(-1);
			}
			
			if (this.status.get(statusStr) != null) {
				ticket.setStatus(this.status.get(statusStr).intValue());
			} else {
				ticket.setStatus(-1);
			}

			ticketList = dao.findTicket(ticket, fetchRowSize, currentRow);

			JSONObject jsonObject = convertObject(ticketList);
			logger.log(Level.INFO, "Json String " + jsonObject.toString());
			
//			taskResponse.setResponseType(ResponseType.JSON);
			taskResponse.setResponseText(jsonObject.toString());

			logger.log(Level.INFO, "searchTicket POST XHR Request called");
			
			/* MySql Pagination Test */
			/*ticketList = dao.getAllTickets(id);
			for (Ticket t: ticketList) {
				logger.log(Level.INFO, String.valueOf(t.getId()));
			}*/

		} else {
			request.setAttribute("tableView", "tableView");
		}
		logger.log(Level.INFO, "searchTicket GET called" + String.valueOf(++callTime));
		//taskResponse.addForwardTask(VBossProperties.getInstance(), "admin.ViewDefinePrivilege");
		return taskResponse;
	}

I had tried debug with firebug which found nothing and check the Json string using JSONINT and it say it is a valid json. What is the reason that cause this error? Please help. Thanks.