Created Unassigned: RounderCornersExtender no longer works [27479]

I downloaded and installed the new AJAX control toolkit (version 4.1.7.725) released in July 2013.

None of the rounder corner extenders (RCE) I have in my ASP.NET application work anymore.

When I load the page with a rounded corner extender on it, I get this error:

"Unhandled exception at line 5906, column 5 in http://localhost:58660/Private/English/LandingPages/Owner_e.aspx?BPID=143&RK=0

0x800a138f - JavaScript runtime error: Unable to get property 'UI' of undefined or null reference"

Here is the line that is highlighted that causes the error:

"$create(Sys.Extended.UI.RoundedCornersBehavior, {"BorderColor":"#CCCCCC","Radius":20,"id":"ctl00_PageBody_ucProperties_e_rcePropertiesCalendar"}, null, null, $get("ctl00_PageBody_ucProperties_e_pnlPropertiesCalendar"));"

I can't possibly be the only person with this problem ... does anyone have a solution?

Erik