Error In ScriptResource.axd: "Object Of Type 'Sys.Extended.UI.AccordionBehavior' Cannot Be Converted To Type 'Sys.UI.Behavior'

I have an asp.net web site where I'm trying to resolve what looks like a problem with ASP.NET AJAX:

Microsoft JScript runtime error:
Sys.ArgumentTypeException: Object of
type
'Sys.Extended.UI.AccordionBehavior'
cannot be converted to type
'Sys.UI.Behavior'. Parameter name:
instance

I've googled around a lot, and looked at the code presented (see below, no idea what it means) but no luck.

I've tried changing scriptmanger scriptmode to release, and a bunch of other things too.

Details:

Visual Studio 2010
ASP.NET 4.0
Ajax control toolkit 4.0
jQuery 1.4.2
jQuery UI 1.8.5

JS Code the error occurs in, inside ScriptResource.axd:

dispose:function()
{
var c=this;
b.UI.Behavior.callBaseMethod(c,eb); --------- this line
var d=c._element;
if(d)
{
var f=c.get_name();
if(f)
d[f]=a;
var e=d._behaviors;
Array.remove(e,c);
if(!e.length)
d._behaviors=a;
delete c._element
}
}

Declaration of accordion control:

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="atlas" %>
(snip)
<atlas:Accordion ID="menu" runat="server" SelectedIndex="0"
ContentCssClass="accordionContent" FadeTransitions="true" FramesPerSecond="30"
TransitionDuration="250" AutoSize="None" Width="270">
</atlas:Accordion>

*Update: *
Added accordion control declaration as requested, is there anything else I can add that could be useful?

Does anyone have any idea about this error at all - am I right in thinking it's MS AJAX not playing nicely with other javascript (probably jQuery UI) on the page?