Sys.webforms.pagerequestmanagerservererrorexception: an unknown error occurred while processing the request on the server. The status code returned from the server was: 500

<asp:panel id="PanelPopup" runat="server" backcolor="#C0FFC0" bordercolor="Green" borderstyle="Solid" borderwidth="2px" forecolor="#0000C0">
    <div id="headerdiv"></div>
    <div id="detail"></div>
    <div id="foterdiv"></div>

    <table cellpadding="0" cellspacing="0" style="width: 144px">
    <tbody>
    <tr>
        <td>                                    
            <table cellpadding="0" cellspacing="0" style="width: 272px">
            <tbody>
            <tr>
                <td>
                    <asp:label id="LabelPartNo" runat="server" font-size="10pt" forecolor="#0000C0" text="Part No" width="136px">
                </td>
                <td>
                    <asp:textbox id="txtPartNo" runat="server" width="200px">
                </td>
                <td>
                </td>
                <td>
                </td>
            </tr>
            <tr>
                <td style="height: 24px">
                    <asp:label id="LabelDescription" runat="server" font-size="10pt" forecolor="#0000C0" text="Description" width="136px">
                </td>
                <td style="height: 24px">
                    <asp:textbox id="txtDescription" runat="server" width="200px">
                </td>
                <td style="height: 24px">
                </td>
                <td style="height: 24px">
                </td>
            </tr>
            <tr>
                <td>
                    <asp:label id="LabelPackage" runat="server" font-size="10pt" forecolor="#0000C0" text="Package" width="136px">
                </td>
                <td>
                    <asp:dropdownlist id="cmbPackage" runat="server" width="205px">
                </td>
                <td>
                </td>
                <td>
                </td>
            </tr>
            </tbody>
            </table>
        </td>
    </tr>
    <tr>
        <td>
        </td>
    </tr>
    <tr>
        <td style="height: 24px">
            <asp:button id="MPEOK" runat="server" text="ADD NEW" width="112px" onclick="MPEOK_Click" visible="False">
        </td>
        <td style="height: 24px">
            <asp:button id="cmdUpdate" runat="server" text="SUBMIT" width="112px" onclick="cmdUpdate_Click">
        </td>
        <td style="height: 24px">
            <asp:button id="cmdDelete" runat="server" text="DELETE" width="112px" onclick="cmdDelete_Click" visible="False">
        </td>
        <td style="width: 81px; height: 24px">
            <asp:button id="MPECancel" runat="server" text="Cancel" width="80px">
        </td>
    </tr>
    </tbody>
    </table>

    <asp:button runat="server" id="btnShowModalPopup" style="display:none" width="152px">

    <cc1:modalpopupextender id="MPECaseDetails" runat="server" popupcontrolid="PanelPopup" targetcontrolid="btnShowModalPopup" popupdraghandlecontrolid="PanelPopup" cancelcontrolid="cmdUpdate" backgroundcssclass="modalBackground">

    <cc2:calendarextender id="ToDate" runat="server" behaviorid="ToTime" format="dd-MMM-yyyy" onclientdateselectionchanged="ToDate" targetcontrolid="txtReceivedDate">