Microsoft JScript runtime error

hello,

i recently added page cache to my web application and it seemed to uncover a problem with the TookkitScriptManager.   on a page load everything looks good, however, with a page refresh/postback, i receive this error: 

"Microsoft JScript runtime error: Unable to get value of the property 'UI': object is null or undefined. Break/Continue/Ignore"

on line:

 Sys.Application.add_init(function() {

    $create(Sys.Extended.UI.RoundedCornersBehavior, {"BorderColor":"#CCCCCC","id":"RoundedCornersExtender3"}, null, null, $get("LoginDisplay"));

});

when i remove the page cache from the default page, the ajax page works as expected.

default.aspx

<%@ OutputCache CacheProfile="StandardCache60min" %>

web.confg

<caching><outputCacheSettings><outputCacheProfiles><add name="StandardCache60min" duration="60000" varyByParam="none" varyByCustom="browser"/></outputCacheProfiles></outputCacheSettings></caching>

i am wondering what the two NULLs are looking for.  any ideas?

thanks!