Cannot trigger update panel from event from cascadingdropdowns

I have a series of cascadingdropdowns. The last dropdpwn has a an autopostback, so that when it's index changes, it can trigger an update panel update.
However when I try it, I get an error;
"Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/>

I should add that if I set EnableEventValidation="false" for the page, I do not get an exception, but also nothing seems to happen either.

<fieldset style="padding:3px">
      <legend>Business Level </legend>  
        <h2>Company</h2>
        <asp:DropDownList ID="ddlCompany" width="99%" runat="server" 
            ToolTip="Company" >
        </asp:DropDownList>
        <h2>Business</h2>
        <asp:DropDownList ID="ddlBusiness" width="99%" runat="server" 
            ToolTip="Business" >
        </asp:DropDownList>
        <h2>Business Streams</h2>
        <asp:DropDownList ID="ddlBusiness_Streams" width="99%" runat="server" 
            ToolTip="Business Streams" >
        </asp:DropDownList>
        <h2>Operating Group</h2>
        <asp:DropDownList ID="ddlOperating_Groups" width="99%" runat="server"
            ToolTip="Operating Group">
        </asp:DropDownList>
        <h2>Business Unit</h2>
        <asp:DropDownList ID="ddlBusiness_Units"  width="99%" runat="server" 
            ToolTip="Business Unit">
        </asp:DropDownList>
        <h2>Division</h2>
        <asp:DropDownList ID="ddlDivisions"  width="99%" runat="server" 
            ToolTip="Division">
        </asp:DropDownList> 
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server"
        TargetControlID="ddlCompany" Category="Company" PromptText="Select a company" 
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetCompanyLevels" />
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown2" runat="server"
        TargetControlID="ddlBusiness" Category="Business" PromptText="Select a business" 
        ParentControlID="ddlCompany"
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetBusinessLevels" />
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown3" runat="server"
        TargetControlID="ddlBusiness_Streams" Category="BusinessStream" PromptText="Select a business stream" 
        ParentControlID="ddlBusiness"
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetBusinessStreamLevels" />
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown4" runat="server"
        TargetControlID="ddlOperating_Groups" Category="OperatingGroup" PromptText="Select an operating group" 
        ParentControlID="ddlBusiness_Streams"
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetOperatingGroupLevels" />
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown5" runat="server"
        TargetControlID="ddlBusiness_Units" Category="BusinessUnit" PromptText="Select a business unit" 
        ParentControlID="ddlOperating_Groups"
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetBusinessUnitLevels" />
        <ajaxToolkit:CascadingDropDown ID="CascadingDropDown6" runat="server"
        TargetControlID="ddlDivisions" Category="Division" PromptText="Select a devision" 
        ParentControlID="ddlBusiness_Units"
        ServicePath="../WebServicesBusinessLevels.asmx" ServiceMethod="GetDivisionLevels" />
      </fieldset> 
  <div id="maincenter">
    <asp:UpdatePanel runat="server" ID="updProjects" UpdateMode="Conditional">
        <ContentTemplate>
       <cc1:InsertableGrid ID="gvwProjects" runat="server" AllowPaging="True" AutoGenerateColumns="False"
        DataSourceID="odsProjects"  DataKeyNames="Div_No,Project"
          AllowInsert="True"
           onRowCreated="gvwProjects_RowCreated" 
           OnRowInserting="gvwProjects_RowInserting"
           OnRowInserted="gvwProjects_RowInserted"
           OnRowUpdated="gvwProjects_RowUpdated">
          <Columns>
              <asp:CommandField ShowEditButton="True" ShowDeleteButton="True" />
              <asp:BoundField DataField="Project" HeaderText="Project" SortExpression="Project" />
              <asp:BoundField DataField="Name_Long" HeaderText="Name Long" SortExpression="Name_Long" />
              <asp:BoundField DataField="Name_Short" HeaderText="Name Short" SortExpression="Name_Short" />
              <asp:BoundField DataField="PM_NetworkID" HeaderText="PM NetworkId" SortExpression="PM_NetworkID" />
          </Columns>
      </cc1:InsertableGrid>
   
      <asp:ObjectDataSource ID="odsProjects" runat="server" 
          SelectMethod="GetProjects_Manual"
          TypeName="Biz.Projects"
          DeleteMethod="Delete_Project"
          InsertMethod="Insert_Project"
          UpdateMethod="Update_Project">
          <SelectParameters>
              <asp:ControlParameter ControlID="ddlDivisions" Name="_Div_No" PropertyName="SelectedValue"
                  Type="Int32" />
          </SelectParameters>
          <DeleteParameters>
              <asp:ControlParameter ControlID="ddlDivisions" Name="Div_No" PropertyName="SelectedValue"
                  Type="Int32" />
              <asp:Parameter Name="Project" Type="string" />
          </DeleteParameters>
          <UpdateParameters>
              <asp:ControlParameter ControlID="ddlDivisions" Name="Div_No" PropertyName="SelectedValue"
                  Type="Int32" />
              <asp:Parameter Name="Project" Type="string" />
              <asp:Parameter Name="Name_Long" Type="string" />
              <asp:Parameter Name="Name_Short" Type="string" />
              <asp:Parameter Name="PM_NetworkID" Type="string" />
          </UpdateParameters>
          <InsertParameters>
              <asp:ControlParameter ControlID="ddlDivisions" Name="Div_No" PropertyName="SelectedValue"
                  Type="Int32" />
              <asp:Parameter Name="Project" Type="string" />
              <asp:Parameter Name="Name_Long" Type="string" />
              <asp:Parameter Name="Name_Short" Type="string" />
              <asp:Parameter Name="PM_NetworkID" Type="string" /> 
          </InsertParameters>
      </asp:ObjectDataSource>
      &nbsp;
        <asp:Label ID="lblMessage" runat="server" ForeColor="Red" ></asp:Label>
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="ddlDivisions" EventName="selectedindexchanged" />
        </Triggers>
    </asp:UpdatePanel>
  </div>

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