Update panels cause javascript error: "Microsoft JScript runtime error: member not found"

I use a dropdown list with autorun enabled. When I change the value in the update panel, it gives a javascript error as well Microsoft JScript runtime error: Member not found. I am using the main page.

Error Location:

The error occurs when "theForm.submit ();"

<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['ctl01'];
if (!theForm) {
    theForm = document.ctl01;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
//]]>
</script>

Asp.Net/HTML:

    <asp:ScriptManager ID="manager" runat="server" ></asp:ScriptManager>
    <asp:UpdatePanel ID="platformOutputTypes" runat="server" UpdateMode="Always" >
        <ContentTemplate >
            <p>
                <label>Platform</label>
                <asp:DropDownList ID="platform" AutoPostBack="true" runat="server" onselectedindexchanged="PlatformSelectedIndexChanged" ></asp:DropDownList>
            </p>

            <asp:CheckBoxList TextAlign="Left" ID="reportOutputTypes" runat="server" />
        </ContentTemplate>
   </asp:UpdatePanel>

I also use jQuery on the page.