When adding a webpart to my ASPX page using the Portal Engine functionality, if I enabled the Ajax/Update Panel feature I kept getting the error message: "The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload..."

Since I was using a webpart that was pre-built modifing the code so that it didn't display any content on the Design tab wasn't working (as referenced on http://devnet.kentico.com/docs/devguide/index.html?developing_web_parts.htm).

After going back and forth with a patient support guru (Thanks Radek!) I realized I had placed my ajaxToolkit control in the wrong place on my masterpage -- it should have been at the top with the cms:CMSPortalManager, whereas I had it at the bottom of the page.

<asp:PlaceHolder runat="server" ID="plcManagers">
        <cms:CMSPortalManager ID="manPortal" runat="server" EnableViewState="false" />
        <ajaxToolkit:ToolkitScriptManager ID="manScript" runat="server" EnableViewState="false"
            ScriptMode="Release" />
    </asp:PlaceHolder>

I am submitting this to provide a solution for Ajax-newbies like myself who might not have known this. Awkward!